Closes#1503
multi_user_bridge.py (2888 lines) had zero test coverage. Any change
to this file could break multi-user functionality with no automated
detection.
23 tests across 5 test classes:
- TestPluginRegistry (7): register, unregister, list, fire hooks,
override returns, thread-safe registration
- TestChatLogIsolation (8): room isolation, rolling buffer, history
with limit/since, get_all_rooms, thread-safe logging, no cross-room
leakage
- TestPresenceManager (6): enter/leave, get_players_in_room, room
isolation, thread-safe concurrent operations
- TestConcurrentUsers (1): multi-user concurrent chat with room
isolation verification (no cross-contamination)
- TestHealthResult/Endpoint (4): data structure tests
