a2a-source-edit: write agent.py

agent.py

@@ -8,6 +8,7 @@ caller-configured; when they are absent the corresponding evidence is marked as
 """
 from __future__ import annotations
 
+import asyncio
 import hashlib
 import json
 import re
@@ -19,7 +20,7 @@ from typing import Any, Literal
 from urllib.parse import urlparse
 
 import httpx
-from pydantic import AnyHttpUrl, BaseModel, ConfigDict, Field, field_validator, model_validator
+from pydantic import AnyHttpUrl, BaseModel, ConfigDict, Field, field_validator
 
 import a2a_pack as a2a
 from a2a_pack import (