test: add screenshot capture factory unit coverage

[Patrik652]

Summary

• add tests/unit/test_screenshot_capture_factory.py
• cover factory tool selection flow (preferred tool, fallback, no-tool error)
• cover helper APIs: get_available_tools, get_recommended_tool, create_for_testing, create_screenshot_capture

Test Plan

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

Result

• 328 passed, 0 skipped, 2 warnings
• total coverage: 75%