fix: adhere to CONTRIBUTING.md — ruff clean + all tests passing

- Move aiomqtt to module-level import (enables proper patch() in tests)
- Remove unused variable (description) in zigbee_service — ruff F841
- Split long line (132 chars) to fit 120 char limit — ruff E501
- Fix import sort order in test files — ruff I001
- Remove unused imports (asyncio, AsyncMock, MagicMock) — ruff F401
- Rename test_mqtt_connection import alias to _test_mqtt_connection
  to avoid pytest fixture name collision (ERROR at setup)
- All 33 backend tests now pass (21 service + 12 router)
- TypeScript typecheck: 0 errors

Co-authored-by: CyberKeys <noreply@openclaw.ai>

backend/tests/test_zigbee_router.py

@@ -7,7 +7,6 @@ from unittest.mock import patch
 import pytest
 from httpx import AsyncClient
 
-
 # ---------------------------------------------------------------------------
 # Fixtures
 # ---------------------------------------------------------------------------