fix: OpenAPI tool _request missing timeout causes ReadTimeout with default httpx 5s

[fuki01]

Bug Description

PR #2872 switched OpenAPI tools from requests (sync) to httpx.AsyncClient (async) in rest_api_tool.py. However, the new _request function does not specify a timeout parameter:

async def _request(**request_params) -> httpx.Response:
  async with httpx.AsyncClient(
      verify=request_params.pop("verify", True)
  ) as client:
    return await client.request(**request_params)

• requests has no default timeout (waits indefinitely)
• httpx.AsyncClient has a default timeout of 5 seconds

This means API calls that previously worked fine now fail with httpx.ReadTimeout if they take longer than 5 seconds.

Steps to Reproduce

1. Configure an OpenAPI tool that calls a slower external API
2. Upgrade from google-adk 1.23.0 to 1.24.0
3. Execute a tool call that takes more than 5 seconds

Expected Behavior

The API call should succeed as it did in 1.23.0 with requests.

Actual Behavior

httpx.ReadTimeout is raised after 5 seconds.

Suggested Fix

Add an explicit timeout to httpx.AsyncClient in _request. The appropriate value is up to the ADK team, but the current implicit 5s default is too short for many real-world API calls.

Environment

• google-adk: 1.24.0+
• Python: 3.13

[wyf7107]

@seanzhougoogle do you mind take a look?