Copy: preserve the block when its entire text is selected

[ellatrix]

What?

Fixes #48040. See #53076. Copying the entire text of a heading and pasting it into an empty paragraph now produces a heading. Pasting it into existing text inserts the words inline instead of splitting the paragraph.

Why?

Selecting all of a heading's text and copying put bare text on the clipboard, so the heading level was lost on paste. Word and Google Docs both preserve the heading in the empty-target case (see the survey in the issue). The previous attempt, #48254, carried the tag name on the rich text clipboard and was reverted (#54301) after regressing copy/paste in nested rich texts like image captions.

How?

1. Copy: when the selection spans a block's entire text and the editable element is the block element itself, the block is copied through the existing setClipboardBlocks path. Fields within larger blocks, like captions, are not the block element and keep the native copy.
2. Paste: pasting over an entirely selected block replaces it, like pasting into an empty block.
3. Rich text's copy handler respects event.defaultPrevented, so it no longer overwrites the clipboard after the block editor claimed the copy.

One expectation is deliberately updated: pasting a paragraph with line breaks into a list creates one item per line, following the paragraph-to-list transform. The previous single-item-with-<br> expectation dated from the multiline list, where item blocks did not exist (#35416).

Testing Instructions

1. Add a heading, select all its text (Cmd+A), copy.
2. Paste into an empty paragraph: a heading appears; previously plain text.
3. Paste into a paragraph with text: the words are inserted at the caret; no split.
4. Copy part of the heading, or a full image caption: pastes as text, unchanged.

Covered by a regression test; fails on trunk.

Use of AI Tools

Written with Claude Code, reviewed and tested by me.

[github-actions]

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Co-authored-by: ellatrix <ellatrix@git.wordpress.org>
Co-authored-by: richtabor <richtabor@git.wordpress.org>
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[github-actions]

Size Change: +92 B (0%)

Total Size: 7.76 MB

📦 View Changed

Filename	Size	Change
build/scripts/block-editor/index.min.js	428 kB	+87 B (+0.02%)
build/scripts/rich-text/index.min.js	14.8 kB	+5 B (+0.03%)

_{compressed-size-action}

[github-actions]

Flaky tests detected in 7ee729c.
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/30584667843
📝 Reported issues:

• [Flaky Test] does not dirty related template parts or navigation menus when rendered in the post editor #79294 in /test/e2e/specs/editor/blocks/navigation-passive-rendering.spec.js

[github-actions]

Warning: Type of PR label mismatch

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• Required label: Any label starting with [Type].
• Labels found: [Package] Rich text, [Package] Block editor, Backport to WP 7.1 Beta/RC.

Read more about Type labels in Gutenberg. Don't worry if you don't have the required permissions to add labels; the PR reviewer should be able to help with the task.

[github-actions]

I just cherry-picked this PR to the wp/7.1 branch to get it included in the next release: 6553823