PR #8361 - simstudioai/sim

Summary

Chat find closed and cleared its query when a pending chat adopted its persisted id (undefined → id), although that's the same conversation. It now resets only when an existing scope changes
Footnotes made the find index and the rendered DOM disagree on match counts and order: the index kept footnote definitions, while the DOM showed the marker, the moved definitions, a visually hidden heading, and back-links. Footnote definitions are now left out of the index, and [data-footnote-ref] / [data-footnotes] are excluded from the DOM walk. The chat's link renderer now forwards data-footnote-ref so the marker can be excluded
Mention chips:
- slash_command contexts render with the / prefix
- the token match accepts trailing punctuation via a lookahead, so @Workflow, and back-to-back repeats (@A @A) now match, while @Workflow-2 still doesn't match @Workflow
- overlapping matches resolve to the longest chip (@report vs @report.pdf, Workflow vs Workflow 2)
The @/blocks/integration-matcher test mock moves to one central @sim/testing mock used by all four suites
Responds to review threads on #8356

use-chat-find.test.tsx renders real ChatContent markup: find survives undefined → id but resets on a chat switch, and a footnoted message's match count equals its highlighted ranges. Each fails with its guard reverted
utils.test.ts: punctuation, repeats, slash commands, the prefix boundary, and longest-overlap cases. The behavior cases fail on staging
Cleanup passes (effects, state, memo, callback, React Query, EMCN, URL state, comments, test audit) found nothing
Home, chat, and composer suites (238 tests), type-check, lint, and check:audits pass

Code follows project style guidelines
Self-reviewed my changes
Tests added/updated and passing
No new warnings introduced
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