Notes: Cancel in-flight hover highlight when focus leaves a note thread

[Mamaduka]

What?

Fixes a minor race condition that is unnoticeable to users but causes an e2e test to fail from time to time.

A note thread highlights its related block from two places: hover (debounced 50ms) and focus/blur (immediate). A hover toggle scheduled but not yet fired could land after the blur handler had already cleared the highlight, re-highlighting the
block. Nothing undoes it until the pointer moves, so the block stays stuck in the highlighted state.

This also made the should collapse a note when the focus moves outside the note e2e test flaky (~3 in 8 locally): whether it failed depended purely on whether the test's remaining steps finished inside that 50ms window.

I originally swapped Shift+Tab for a direct .focus() call, but that didn't fix it.

How

debouncedToggleBlockHighlight.cancel() before each immediate write, so a focus-driven toggle always supersedes a pending hover one.

Testing Instructions

This is hard to reproduce manually. Running the mentioned tests multiple times on the trunk will fail.

Use of AI Tools

Assisted by Claude.

[github-actions]

Size Change: +9 B (0%)

Total Size: 7.75 MB

📦 View Changed

Filename	Size	Change
build/scripts/editor/index.min.js	499 kB	+9 B (0%)

_{compressed-size-action}

[github-actions]

Flaky tests detected in 790efc5.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/30332761949
📝 Reported issues:

• [Flaky Test] Should insert content using the global inserter #78924 in /test/e2e/specs/widgets/editing-widgets.spec.js

[github-actions]

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[Mamaduka]

This is a follow-up to #80222. So, labeling for backporting.

[github-actions]

I just cherry-picked this PR to the wp/7.1 branch to get it included in the next release: 254b058