fix: add explicit timeout to OpenAPI tool httpx client

The migration from `requests` to `httpx.AsyncClient` in PR #2872 did not
specify a timeout parameter. Since `requests` defaults to no timeout
while `httpx` defaults to 5 seconds, this causes `ReadTimeout` errors
for API calls that take longer than 5 seconds.

Set timeout to `None` to restore the previous behavior.

Closes #4431

Author: fuki01

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

@@ -560,6 +560,7 @@ def __repr__(self):
 
 async def _request(**request_params) -> httpx.Response:
   async with httpx.AsyncClient(
-      verify=request_params.pop("verify", True)
+      verify=request_params.pop("verify", True),
+      timeout=None,
   ) as client:
     return await client.request(**request_params)