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/common/relay_info.go

@@ -154,6 +154,13 @@ type RelayInfo struct {
 	UseRuntimeHeadersOverride             bool
 	ParamOverrideAudit                    []string
 
+	// UpstreamRequestBodySize is the byte size of the marshaled upstream request
+	// body. It is set when the body is wrapped in a BodyStorage (see
+	// relay/common/outbound_body.go), so that DoApiRequest can populate
+	// http.Request.ContentLength manually (net/http only auto-detects it for
+	// *bytes.Reader/Buffer/strings.Reader). 0 means "let net/http decide".
+	UpstreamRequestBodySize int64
+
 	PriceData types.PriceData
 
 	// TieredBillingSnapshot is a frozen snapshot of tiered billing rules