feat(QTI): add Gap Match interaction viewer - #15214

Summary

This PR adds the QTI Gap Match interaction.
Gap match asks a learner to fill blanks in a passage from a pool of words. The fixtures for it already existed but I added extra examples for the answer specific pools.

What it does

• Renders qti-gap-match-interaction with a response pool above the passage and inline blanks in the text
• Fill a gap by clicking a word then a gap, or a gap then a word
• Drag a word into a gap, between gaps, or back to the pool
• Tab reaches each gap and the arrow keys step through its options, so nothing needs a mouse
• Clicking a filled gap empties it; dropping onto one replaces what's there
• Restores a saved answer and reports it back for scoring

Answer-specific distractor pools

A gap can be fed by its own set of words instead of the whole pool. Picking up a word highlights the gaps that would take it
Behavior description:

• While dragging, gaps that would reject the word are dimmed
• Gaps that already hold a word aren't highlighted, though they can still be replaced
• An item with no match-group behaves exactly as before, one pool feeding every gap

Notable details

• Gaps are found by walking the passage, since a gap can sit anywhere the author puts it, inside a paragraph, a table cell, a quotation
• Gap match writes its pairs word-first, the opposite of match interaction, so useMatchRows gained an option for which end comes first. Match interaction is unaffected.
• Gap.vue takes its attributes over rather than inheriting them. An inherited class replaces the component's own root class instead of joining it, which would break both the gap styling and the width classes.
• qti-input-width-N sizes a blank to roughly N characters

$\color{rgba(0, 128, 0)}{\textsf{Gap match inside a table example:}}$

References

• fixes Product Issue: Gap Match QTI Interaction #15213
• based on feat(QTI): add associate and match interaction viewers #15151

Reviewer guidance

1. Navigate to http://localhost:8000/en/learn/#/qti_sandbox
2. All of the gap match examples in the sandbox are supported and render
   but these examples don't do all what the name implies, some of these were dropped:
   • Positioning: qti-choices-top/bottom/left/right are ignored, so all four pools sit above their passage instead of beside it.
   • Container Width: data-choices-container-width is ignored. The apparent width difference comes from the fixture's own qti-layout-col5 / qti-layout-col9 wrappers.
   • Custom Messages: data-max-selections-message and data-min-selections-message are ignored, and there's no notice yet at all, so nothing is shown when a limit is hit.

AI usage

All unit tests are written by Claude in addition to the extra fixture I added