feat: prevent metadata from overriding model fields

relay/channel/task/taskcommon/helpers.go

@@ -17,6 +17,8 @@ func UnmarshalMetadata(metadata map[string]any, target any) error {
 	if metadata == nil {
 		return nil
 	}
+	// Prevent metadata from overriding model fields to avoid billing bypass.
+	delete(metadata, "model")
 	metaBytes, err := common.Marshal(metadata)
 	if err != nil {
 		return fmt.Errorf("marshal metadata failed: %w", err)