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.

sdk/cliproxy/usage/manager.go

@@ -2,6 +2,7 @@ package usage
 
 import (
 	"context"
+	"net/http"
 	"strings"
 	"sync"
 	"time"
@@ -24,6 +25,8 @@ type Record struct {
 	Failed      bool
 	Fail        Failure
 	Detail      Detail
+	// ResponseHeaders stores a snapshot of upstream response headers for usage sinks.
+	ResponseHeaders http.Header
 }
 
 // Failure holds HTTP failure metadata for an upstream request attempt.