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/runtime/executor/helps/logging_helpers.go

@@ -102,6 +102,7 @@ func RecordAPIRequest(ctx context.Context, cfg *config.Config, info UpstreamRequ
 
 // RecordAPIResponseMetadata captures upstream response status/header information for the latest attempt.
 func RecordAPIResponseMetadata(ctx context.Context, cfg *config.Config, status int, headers http.Header) {
+	logging.SetResponseHeaders(ctx, headers)
 	if cfg == nil || !cfg.RequestLog {
 		return
 	}
@@ -227,6 +228,7 @@ func RecordAPIWebsocketRequest(ctx context.Context, cfg *config.Config, info Ups
 
 // RecordAPIWebsocketHandshake stores the upstream websocket handshake response metadata.
 func RecordAPIWebsocketHandshake(ctx context.Context, cfg *config.Config, status int, headers http.Header) {
+	logging.SetResponseHeaders(ctx, headers)
 	if cfg == nil || !cfg.RequestLog {
 		return
 	}
@@ -250,6 +252,7 @@ func RecordAPIWebsocketHandshake(ctx context.Context, cfg *config.Config, status
 
 // RecordAPIWebsocketUpgradeRejection stores a rejected websocket upgrade as an HTTP attempt.
 func RecordAPIWebsocketUpgradeRejection(ctx context.Context, cfg *config.Config, info UpstreamRequestLog, status int, headers http.Header, body []byte) {
+	logging.SetResponseHeaders(ctx, headers)
 	if cfg == nil || !cfg.RequestLog {
 		return
 	}