fix: use 10-minute default timeout instead of None

Set a generous 600s default to prevent indefinite hangs while
still accommodating slow API responses.

Author: fuki01

src/google/adk/tools/openapi_tool/openapi_spec_parser/rest_api_tool.py

@@ -561,6 +561,6 @@ def __repr__(self):
 async def _request(**request_params) -> httpx.Response:
   async with httpx.AsyncClient(
       verify=request_params.pop("verify", True),
-      timeout=request_params.pop("timeout", None),
+      timeout=request_params.pop("timeout", httpx.Timeout(600.0)),
   ) as client:
     return await client.request(**request_params)