fix: report missing anywidget for JupyterChart

[dk3yyyy]

Summary

• add the missing enable_offline() API to the JupyterChart fallback used when anywidget is unavailable
• detect the optional dependency with find_spec() and expose the real class during type checking
• remove obsolete type ignores from the Jupyter renderer
• add regression coverage for the missing-dependency behavior and restore imported module state after the test

Fixes #3801

Testing

• uv run task test
• uv run pytest tests/test_jupyter_chart.py -q --numprocesses=1
• uv run pytest tests/vegalite/v6/test_renderers.py -q --numprocesses=1
• verified a base-only installation without anywidget reports the intended ImportError for alt.JupyterChart, enable_offline(), and the jupyter renderer

[mattijn]

Thanks for this PR @dk3yyyy! Would it be possible to adapt your contribution to have the test run without a pytest.monkeypach()? I think we have a CI run that only does the base installation or at least options to have certain optional deps not included. Thanks again for this PR!

[dk3yyyy]

Thanks @mattijn! I’ve updated the test to avoid pytest.monkeypatch and module reloading. It now runs directly against alt.JupyterChart when anywidget is unavailable, using the existing Python 3.10 CI jobs where anywidget is explicitly uninstalled.

The test covers both JupyterChart.enable_offline() and construction through the public API. I also verified it locally in the equivalent no-anywidget setup: 1 passed, with the 5 tests requiring anywidget correctly skipped.

[mattijn]

Thanks @dk3yyyy! CI is happy with your latest commit. Thanks!

[dk3yyyy]

Thanks for the review and merge, @mattijn!