feat(runtime): track upstream response headers in logging and usage reporting

- Added APIs to store, retrieve, and clone upstream response headers in context for detailed logging.
- Updated `RecordAPIResponseMetadata`, `RecordAPIWebsocketHandshake`, and related methods to capture response headers.
- Extended `UsageReporter` to include response headers in published usage records.
- Enhanced payload tests to validate response headers' integrity and persistence.
- Refactored `usage.Record` to support optional `ResponseHeaders` field.

internal/redisqueue/plugin.go

@@ -3,6 +3,7 @@ package redisqueue
 import (
 	"context"
 	"encoding/json"
+	"net/http"
 	"strings"
 	"time"
 
@@ -71,13 +72,14 @@ func (p *usageQueuePlugin) HandleUsage(ctx context.Context, record coreusage.Rec
 	fail := resolveFail(ctx, record, failed)
 
 	detail := requestDetail{
-		Timestamp: timestamp,
-		LatencyMs: record.Latency.Milliseconds(),
-		Source:    record.Source,
-		AuthIndex: record.AuthIndex,
-		Tokens:    tokens,
-		Failed:    failed,
-		Fail:      fail,
+		Timestamp:       timestamp,
+		LatencyMs:       record.Latency.Milliseconds(),
+		Source:          record.Source,
+		AuthIndex:       record.AuthIndex,
+		Tokens:          tokens,
+		Failed:          failed,
+		Fail:            fail,
+		ResponseHeaders: record.ResponseHeaders,
 	}
 
 	payload, err := json.Marshal(queuedUsageDetail{
@@ -108,13 +110,14 @@ type queuedUsageDetail struct {
 }
 
 type requestDetail struct {
-	Timestamp time.Time  `json:"timestamp"`
-	LatencyMs int64      `json:"latency_ms"`
-	Source    string     `json:"source"`
-	AuthIndex string     `json:"auth_index"`
-	Tokens    tokenStats `json:"tokens"`
-	Failed    bool       `json:"failed"`
-	Fail      failDetail `json:"fail"`
+	Timestamp       time.Time   `json:"timestamp"`
+	LatencyMs       int64       `json:"latency_ms"`
+	Source          string      `json:"source"`
+	AuthIndex       string      `json:"auth_index"`
+	Tokens          tokenStats  `json:"tokens"`
+	Failed          bool        `json:"failed"`
+	Fail            failDetail  `json:"fail"`
+	ResponseHeaders http.Header `json:"response_headers,omitempty"`
 }
 
 type tokenStats struct {