fix(core): restore fallback serialization in content-utils to avoid throwing

packages/core/src/agent/content-utils.ts

@@ -93,12 +93,13 @@ export function contentPartsToGeminiParts(content: ContentPart[]): Part[] {
         // References are converted to text for the model
         result.push({ text: part.text });
         break;
-      default: {
-        ((x: never) => {
-          throw new Error(`Unhandled ContentPart type: ${JSON.stringify(x)}`);
-        })(part);
+      default:
+        debugLogger.warn(
+          `Unhandled ContentPart type: ${JSON.stringify(part)} fallback to serialization`,
+        );
+        // Serialize unknown ContentPart variants instead of dropping them
+        result.push({ text: JSON.stringify(part) });
         break;
-      }
     }
   }
   return result;