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/image_handler.go

@@ -77,7 +77,14 @@ func ImageHelper(c *gin.Context, info *relaycommon.RelayInfo) (newAPIError *type
 			}
 
 			logger.LogDebug(c, "image request body: %s", jsonData)
-			requestBody = bytes.NewBuffer(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
 		}
 	}