Preserve nested list when deleting a selection across sibling list items

[ellatrix]

What?

Fix a nested list being dropped when deleting a text selection that spans two sibling list items.

Why?

Deleting a multi-block text selection merged only block attributes, so inner blocks (e.g. a nested list) on the block merged away were lost. Backspace dropped them; Delete only kept them because the target happened to be the item that owned the nested list.

How?

In __unstableDeleteSelection, carry both blocks' inner blocks (in document order) onto the surviving merged block.

Testing Instructions

1. Create a list with items "ab" and "cd", and nest a "test" item under "cd".
2. Select text spanning "ab" and "cd" (e.g. from inside "ab" to inside "cd").
3. Press Backspace.
4. The items merge and the nested "test" list is preserved.

Testing Instructions for Keyboard

Make the selection in step 2 with shift+arrow keys.

Use of AI Tools

Authored with Claude Code (Claude Opus 4.7).

[github-actions]

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Co-authored-by: ellatrix <ellatrix@git.wordpress.org>

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[github-actions]

Size Change: +9 B (0%)

Total Size: 8.44 MB

📦 View Changed

Filename	Size	Change
build/scripts/block-editor/index.min.js	380 kB	+9 B (0%)

_{compressed-size-action}

[github-actions]

Flaky tests detected in fd6d05f.
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/27132728887
📝 Reported issues:

• [Flaky Test] Should insert content using the global inserter #78924 in /test/e2e/specs/widgets/editing-widgets.spec.js
• [Flaky Test] shouldn't conflict with server-side autosave #78706 in /test/e2e/specs/editor/various/autosave.spec.js