fix(live): keep receive open across turns

[MukundaKatta]

Summary

• stop AsyncLiveSession.receive() from exiting after the first turn_complete event
• keep receive() usable for multi-turn live sessions that iterate once for the whole connection
• add a regression test that mocks _receive() across multiple turns

Testing

• python3 -m py_compile google/genai/live.py google/genai/tests/live/test_live_response.py
• python3 -m pytest google/genai/tests/live/test_live_response.py -k receive (fails locally: pytest_asyncio is not installed in this environment)
• python3 smoke check for the mocked receive loop (blocked locally because google.auth is not installed in this environment)