test: raise monitoring session manager coverage to 97%

[Patrik652]

Summary

• add tests/unit/test_monitoring_session_manager_core.py
• cover start/stop/pause/resume edge paths, capture-loop branch handling, and idle-seconds detection branches
• improve stability confidence for auto-monitoring control flow

Test Plan

• PYTHONPATH=. .venv/bin/ruff check src/services src/cli tests/unit/test_monitoring_session_manager_core.py
• PYTHONPATH=. .venv/bin/mypy src/services src/cli
• PYTHONPATH=. .venv/bin/pytest -q

Result

• 378 passed, 0 skipped, 2 warnings
• total coverage: 87%
• src/services/monitoring_session_manager.py: 97%