test: harden screenshot factory and typing checks

[Patrik652]

Summary

• add targeted unit tests for screenshot capture factory implementation branches
• tighten type annotations in exceptions to satisfy no_implicit_optional
• make entities typing Python 3.8-compatible and document intentional A003 fields via per-file ignore

Test Plan

• . ./.venv/bin/activate && ruff check src/services src/cli tests/unit/test_screenshot_capture_factory.py src/lib/exceptions.py src/models/entities.py
• . ./.venv/bin/activate && mypy src/services src/cli
• . ./.venv/bin/activate && PYTHONPATH=. pytest -q