perf: reduce heap residency for large base64 relay requests

Three layered optimizations targeting Gemini-style 5MB base64 payloads where
RSS could balloon to tens of GB under concurrent load:

1. Byte-based param override (relay/common/override.go)
   - Switch legacy/operations hot paths from common.Marshal round-trips and
     map[string]any conversions to gjson/sjson on []byte directly.
   - Avoids cloning 5MB strings during each Set/Delete operation.

2. strings.Builder for Gemini response markdown (relay/channel/gemini/relay-gemini.go)
   - Replace string concatenation + strings.Join when assembling
     "![image](data:...;base64,DATA)" content for inline image responses.
   - Pre-allocates capacity from inline_data byte sizes.

3. Outbound BodyStorage + streaming Decoder (this commit's core)
   - New relay/common/outbound_body.go helper wraps marshaled upstream bodies
     in common.BodyStorage, allowing disk-cache mode to offload jsonData to
     a temp file while waiting for upstream TTFB. The original []byte can
     then be GC'd, removing ~5MB/req of heap residency during the longest
     window of a request.
   - All 7 relay handlers (gemini/claude/responses/embedding/image/compatible/
     rerank) plus chat_completions_via_responses adopt the helper with
     defer closer.Close() and explicit jsonData = nil.
   - relay/common/relay_info.go: new UpstreamRequestBodySize so
     relay/channel/api_request.go can populate req.ContentLength (lost when
     body becomes a type-erased io.Reader).
   - common/gin.go UnmarshalBodyReusable: when storage is disk-backed and
     content-type is JSON, decode via DecodeJson(storage) instead of
     storage.Bytes()+Unmarshal, removing one transient 5MB copy per request.
     memory mode and form/multipart paths unchanged.

relay/channel/api_request.go

@@ -25,6 +25,23 @@ import (
 	"github.com/gorilla/websocket"
 )
 
+// applyUpstreamContentLength populates req.ContentLength when the upstream
+// body is wrapped in a BodyStorage (see relay/common/outbound_body.go).
+//
+// net/http.NewRequest only auto-detects ContentLength for *bytes.Reader,
+// *bytes.Buffer and *strings.Reader. When the body is a type-erased io.Reader
+// (which is the case for ReaderOnly(BodyStorage)), the Content-Length header
+// would otherwise be omitted, forcing chunked transfer encoding and breaking
+// some upstreams that require an explicit Content-Length.
+func applyUpstreamContentLength(req *http.Request, info *common.RelayInfo) {
+	if info == nil {
+		return
+	}
+	if info.UpstreamRequestBodySize > 0 && req.ContentLength <= 0 {
+		req.ContentLength = info.UpstreamRequestBodySize
+	}
+}
+
 func SetupApiRequestHeader(info *common.RelayInfo, c *gin.Context, req *http.Header) {
 	if info.RelayMode == constant.RelayModeAudioTranscription || info.RelayMode == constant.RelayModeAudioTranslation {
 		// multipart/form-data
@@ -297,6 +314,7 @@ func DoApiRequest(a Adaptor, c *gin.Context, info *common.RelayInfo, requestBody
 	if err != nil {
 		return nil, fmt.Errorf("new request failed: %w", err)
 	}
+	applyUpstreamContentLength(req, info)
 	headers := req.Header
 	err = a.SetupRequestHeader(c, &headers, info)
 	if err != nil {
@@ -326,6 +344,7 @@ func DoFormRequest(a Adaptor, c *gin.Context, info *common.RelayInfo, requestBod
 	if err != nil {
 		return nil, fmt.Errorf("new request failed: %w", err)
 	}
+	applyUpstreamContentLength(req, info)
 	// set form data
 	req.Header.Set("Content-Type", c.Request.Header.Get("Content-Type"))
 	headers := req.Header
@@ -522,6 +541,7 @@ func DoTaskApiRequest(a TaskAdaptor, c *gin.Context, info *common.RelayInfo, req
 	if err != nil {
 		return nil, fmt.Errorf("new request failed: %w", err)
 	}
+	applyUpstreamContentLength(req, info)
 	req.GetBody = func() (io.ReadCloser, error) {
 		return io.NopCloser(requestBody), nil
 	}

relay/channel/gemini/relay-gemini.go

@@ -1079,17 +1079,47 @@ func responseGeminiChat2OpenAI(c *gin.Context, response *dto.GeminiChatResponse)
 			FinishReason: constant.FinishReasonStop,
 		}
 		if len(candidate.Content.Parts) > 0 {
-			var texts []string
+			// 使用 strings.Builder 直接累积最终 content，避免:
+			//   1) 每张 inline image 生成一次中间 "![image](...)" 字符串
+			//   2) 末尾 strings.Join 再分配一份等大缓冲
+			// Gemini 图片返回时 InlineData.Data 可能是数 MB 的 base64，
+			// 上述两份临时分配在高并发下会显著放大堆驻留。
+			var content strings.Builder
+			var inlineGrow int
+			for _, part := range candidate.Content.Parts {
+				if part.InlineData != nil {
+					inlineGrow += len(part.InlineData.MimeType) + len(part.InlineData.Data) + 32
+				}
+			}
+			if inlineGrow > 0 {
+				content.Grow(inlineGrow)
+			}
+			appended := 0
+			writeSep := func() {
+				if appended > 0 {
+					content.WriteByte('\n')
+				}
+				appended++
+			}
 			var toolCalls []dto.ToolCallResponse
 			for _, part := range candidate.Content.Parts {
 				if part.InlineData != nil {
 					// 媒体内容
 					if strings.HasPrefix(part.InlineData.MimeType, "image") {
-						imgText := "![image](data:" + part.InlineData.MimeType + ";base64," + part.InlineData.Data + ")"
-						texts = append(texts, imgText)
+						writeSep()
+						content.WriteString("![image](data:")
+						content.WriteString(part.InlineData.MimeType)
+						content.WriteString(";base64,")
+						content.WriteString(part.InlineData.Data)
+						content.WriteByte(')')
 					} else {
 						// 其他媒体类型，直接显示链接
-						texts = append(texts, fmt.Sprintf("[media](data:%s;base64,%s)", part.InlineData.MimeType, part.InlineData.Data))
+						writeSep()
+						content.WriteString("[media](data:")
+						content.WriteString(part.InlineData.MimeType)
+						content.WriteString(";base64,")
+						content.WriteString(part.InlineData.Data)
+						content.WriteByte(')')
 					}
 				} else if part.FunctionCall != nil {
 					choice.FinishReason = constant.FinishReasonToolCalls
@@ -1100,13 +1130,22 @@ func responseGeminiChat2OpenAI(c *gin.Context, response *dto.GeminiChatResponse)
 					choice.Message.ReasoningContent = &part.Text
 				} else {
 					if part.ExecutableCode != nil {
-						texts = append(texts, "```"+part.ExecutableCode.Language+"\n"+part.ExecutableCode.Code+"\n```")
+						writeSep()
+						content.WriteString("```")
+						content.WriteString(part.ExecutableCode.Language)
+						content.WriteByte('\n')
+						content.WriteString(part.ExecutableCode.Code)
+						content.WriteString("\n```")
 					} else if part.CodeExecutionResult != nil {
-						texts = append(texts, "```output\n"+part.CodeExecutionResult.Output+"\n```")
+						writeSep()
+						content.WriteString("```output\n")
+						content.WriteString(part.CodeExecutionResult.Output)
+						content.WriteString("\n```")
 					} else {
 						// 过滤掉空行
 						if part.Text != "\n" {
-							texts = append(texts, part.Text)
+							writeSep()
+							content.WriteString(part.Text)
 						}
 					}
 				}
@@ -1115,7 +1154,7 @@ func responseGeminiChat2OpenAI(c *gin.Context, response *dto.GeminiChatResponse)
 				choice.Message.SetToolCalls(toolCalls)
 				isToolCall = true
 			}
-			choice.Message.SetStringContent(strings.Join(texts, "\n"))
+			choice.Message.SetStringContent(content.String())
 
 		}
 		if candidate.FinishReason != nil {
@@ -1169,7 +1208,25 @@ func streamResponseGeminiChat2OpenAI(geminiResponse *dto.GeminiChatResponse) (*d
 				//Role: "assistant",
 			},
 		}
-		var texts []string
+		// 使用 strings.Builder 直接累积 delta content，避免每张 image / 每个
+		// 文本片段都先 `+` 拼出一份临时 string，再 strings.Join 再拷贝一遍。
+		var content strings.Builder
+		var inlineGrow int
+		for _, part := range candidate.Content.Parts {
+			if part.InlineData != nil {
+				inlineGrow += len(part.InlineData.MimeType) + len(part.InlineData.Data) + 32
+			}
+		}
+		if inlineGrow > 0 {
+			content.Grow(inlineGrow)
+		}
+		appended := 0
+		writeSep := func() {
+			if appended > 0 {
+				content.WriteByte('\n')
+			}
+			appended++
+		}
 		isTools := false
 		isThought := false
 		if candidate.FinishReason != nil {
@@ -1207,8 +1264,12 @@ func streamResponseGeminiChat2OpenAI(geminiResponse *dto.GeminiChatResponse) (*d
 		for _, part := range candidate.Content.Parts {
 			if part.InlineData != nil {
 				if strings.HasPrefix(part.InlineData.MimeType, "image") {
-					imgText := "![image](data:" + part.InlineData.MimeType + ";base64," + part.InlineData.Data + ")"
-					texts = append(texts, imgText)
+					writeSep()
+					content.WriteString("![image](data:")
+					content.WriteString(part.InlineData.MimeType)
+					content.WriteString(";base64,")
+					content.WriteString(part.InlineData.Data)
+					content.WriteByte(')')
 				}
 			} else if part.FunctionCall != nil {
 				isTools = true
@@ -1219,23 +1280,33 @@ func streamResponseGeminiChat2OpenAI(geminiResponse *dto.GeminiChatResponse) (*d
 
 			} else if part.Thought {
 				isThought = true
-				texts = append(texts, part.Text)
+				writeSep()
+				content.WriteString(part.Text)
 			} else {
 				if part.ExecutableCode != nil {
-					texts = append(texts, "```"+part.ExecutableCode.Language+"\n"+part.ExecutableCode.Code+"\n```\n")
+					writeSep()
+					content.WriteString("```")
+					content.WriteString(part.ExecutableCode.Language)
+					content.WriteByte('\n')
+					content.WriteString(part.ExecutableCode.Code)
+					content.WriteString("\n```\n")
 				} else if part.CodeExecutionResult != nil {
-					texts = append(texts, "```output\n"+part.CodeExecutionResult.Output+"\n```\n")
+					writeSep()
+					content.WriteString("```output\n")
+					content.WriteString(part.CodeExecutionResult.Output)
+					content.WriteString("\n```\n")
 				} else {
 					if part.Text != "\n" {
-						texts = append(texts, part.Text)
+						writeSep()
+						content.WriteString(part.Text)
 					}
 				}
 			}
 		}
 		if isThought {
-			choice.Delta.SetReasoningContent(strings.Join(texts, "\n"))
+			choice.Delta.SetReasoningContent(content.String())
 		} else {
-			choice.Delta.SetContentString(strings.Join(texts, "\n"))
+			choice.Delta.SetContentString(content.String())
 		}
 		if isTools {
 			choice.FinishReason = &constant.FinishReasonToolCalls