Handle missing widget state metadata in WidgetsDataTypeFilter

[hpy1968]

Summary

This fixes a KeyError when notebook metadata contains the widget-state mimetype object but that object does not include a nested "state" key.

Reproducer shape

metadata = {
    "widgets": {
        "application/vnd.jupyter.widget-state+json": {
            "version_major": 2,
            "version_minor": 0,
        }
    }
}

When an output includes application/vnd.jupyter.widget-view+json, WidgetsDataTypeFilter currently indexes:

metadata["widgets"][WIDGET_STATE_MIMETYPE]["state"]

and raises:

KeyError: "state"

Fix

This change treats missing "state" the same as unavailable widget state. The filter skips the widget-view mimetype and falls back to the next available display format, such as text/plain.

Tests

Tests added:

• widget metadata without "state" falls back to text/plain
• widget metadata with matching state still prefers widget-view

Tested locally:

• tests/filters/test_widgetsdatatypefilter.py tests/filters/test_datatypefilter.py: 5 passed
• tests/filters: 54 passed, 5 skipped
• full suite: 290 passed, 41 skipped

Related issues

• Fixes "KeyError: 'state'" when trying to convert a notebook from Google Colab #1731
• Related to 'state' KeyError when calling html_exporter.from_notebook_node #2127

Debugging / reproduction note

We reproduced this failure using Retrace, our Python record/replay debugger, to inspect the runtime state at the failing call site.

The replay showed that the notebook metadata contained the widget-state mimetype object, but that object only had version_major and version_minor; the nested "state" key was absent. That is the shape covered by the new test.

The fix and tests above do not depend on Retrace, but the replay was useful for confirming the exact runtime metadata shape behind the KeyError.

For context, we documented the replay/debugging workflow here:
https://github.com/retracesoftware/retracesoftware/tree/main/open-source-debugging/examples/001-nbconvert-1731