fix prototype backend review issues

sdk/agent_session.py

@@ -4,8 +4,6 @@ from dataclasses import dataclass
 from typing import AsyncIterator
 from urllib.parse import parse_qsl, urlencode, urlsplit, urlunsplit
 
-import aiohttp
-
 from sdk.interface import MessageChunk, MessageResponse, PlatformError
 
 
@@ -41,6 +39,8 @@ class AgentSessionClient:
         )
 
     async def stream_message(self, *, thread_key: str, text: str) -> AsyncIterator[MessageChunk]:
+        import aiohttp
+
         async with aiohttp.ClientSession() as session:
             async with session.ws_connect(
                 self._ws_url(thread_key),