Add @pytest.mark.unit to all test classes in test_router_cascade.py so
they are picked up by `tox -e unit`. Expand coverage with new test classes
for _detect_content_type, _transform_messages_for_ollama, Provider
capability methods, _get_fallback_model, cascade_tier filtering, the
generate_with_image convenience method, get_router() singleton, and
_is_provider_available circuit-breaker transitions.
Fixes#1138
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
