Fix xdist timeout/stall hazards in recently changed tests

- Replace leaked real httpx.AsyncClient with AsyncMock in backend service
  targeted tests to prevent event-loop resource leaks on Windows workers.
- Convert asyncio.run() calls to native async/await in async tests to avoid
  nested event-loop conflicts with xdist worker loop management.
- Replace unbounded asyncio.Event().wait() in Gemini ACP cancellation test
  with owned event handle to prevent edge-case hangs.
- Add explicit cwd save/restore in project directory resolution tests to
  prevent cwd bleed across xdist workers.
- Save and restore sys.modules deletions in backend autodiscovery and
  import integration tests to prevent module-cache corruption across workers.
- Add reset_backend_discovery_state() in teardown for clean import fixtures.

Author: Mateusz