test: rewrite test_channel_allowlist to test real WebSocketGateway instead of mock

- Remove MockGateway class that reimplemented production logic
- Test actual WebSocketGateway.start_channels() method via async pytest
- Mock only _create_bot to prevent network connections, all allowlist logic is real
- Add regression test that demonstrates failure if production fix is reverted
- Maintain all original test coverage with proper parametrization
- Tests now verify server.py:1367-1394 allowlist extraction actually works

Co-authored-by: Mervin Praison <MervinPraison@users.noreply.github.com>

Author: praisonai-triage-agent[bot]