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/gemini_handler.go

@@ -1,7 +1,6 @@
 package relay
 
 import (
-	"bytes"
 	"fmt"
 	"io"
 	"net/http"
@@ -165,7 +164,14 @@ func GeminiHelper(c *gin.Context, info *relaycommon.RelayInfo) (newAPIError *typ
 
 		logger.LogDebug(c, "Gemini request body: %s", jsonData)
 
-		requestBody = bytes.NewReader(jsonData)
+		body, size, closer, err := relaycommon.NewOutboundJSONBody(jsonData)
+		if err != nil {
+			return types.NewError(err, types.ErrorCodeConvertRequestFailed, types.ErrOptionWithSkipRetry())
+		}
+		defer closer.Close()
+		jsonData = nil
+		info.UpstreamRequestBodySize = size
+		requestBody = body
 	}
 
 	resp, err := adaptor.DoRequest(c, info, requestBody)
@@ -263,7 +269,14 @@ func GeminiEmbeddingHandler(c *gin.Context, info *relaycommon.RelayInfo) (newAPI
 		}
 	}
 	logger.LogDebug(c, "Gemini embedding request body: %s", jsonData)
-	requestBody = bytes.NewReader(jsonData)
+	body, size, closer, err := relaycommon.NewOutboundJSONBody(jsonData)
+	if err != nil {
+		return types.NewError(err, types.ErrorCodeConvertRequestFailed, types.ErrOptionWithSkipRetry())
+	}
+	defer closer.Close()
+	jsonData = nil
+	info.UpstreamRequestBodySize = size
+	requestBody = body
 
 	resp, err := adaptor.DoRequest(c, info, requestBody)
 	if err != nil {