test: prevent test_resource_limits_set from hanging in GitHub Actions

Set mock_process.pid to None in the resource limits test. Without this, the `finally` block in `run_async` attempts to call `os.killpg` with a fallback `Mock` object (which evaluates to True). In some environments like GitHub Actions, this can inadvertently kill the test runner's process group rather than raising a TypeException, causing the CI job to hang indefinitely.

Co-authored-by: Liang Wu <wuliang@google.com>
PiperOrigin-RevId: 895992429

Author: wuliang229

tests/unittests/tools/test_bash_tool.py

@@ -252,6 +252,7 @@ async def test_resource_limits_set(self, workspace, tool_context_confirmed):
     )
     tool = bash_tool.ExecuteBashTool(workspace=workspace, policy=policy)
     mock_process = mock.AsyncMock()
+    mock_process.pid = None  # Ensure finally block doesn't try to kill it
     mock_process.communicate.return_value = (b"", b"")
     mock_exec = mock.AsyncMock(return_value=mock_process)