Writing flow: forward delete an empty paragraph without breaking apart the next block

[ellatrix]

What?

Fixes #21558. Forward deleting an empty paragraph before a list removes the paragraph and leaves the list intact, caret at the start of the first item. Previously the empty paragraph survived and the list's first item was pulled out and demoted to a paragraph.

Why?

Matches other editors: forward delete joins across the boundary, and an empty line dissolves. Backspace already behaves conventionally in both spots (removes the empty paragraph; strips the bullet at the first item's start) and is unchanged.

How?

The forward branch of onMerge went straight to moveFirstItemUp when the next block has children. Now an unmodified default block is removed instead, and the next block's first leaf is selected, mirroring the existing rule in mergeBlocks and switchToDefaultOrRemove.

Testing Instructions

1. Add an empty paragraph followed by a list.
2. Focus the empty paragraph and press Fn+Backspace (Delete).
3. The paragraph is removed, the list is whole, caret at the start of the first item.

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: draganescu <andraganescu@git.wordpress.org>
Co-authored-by: kathrynwp <zoonini@git.wordpress.org>
Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>
Co-authored-by: bph <bph@git.wordpress.org>
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[github-actions]

Size Change: +36 B (0%)

Total Size: 7.75 MB

📦 View Changed

Filename	Size	Change
build/scripts/block-editor/index.min.js	428 kB	+36 B (+0.01%)

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[ellatrix]

Very low risk and tiny surface area (particular forward delete scenario), added an e2e test, so let's merge and include in 7.1's beta cycle.

[github-actions]

I just cherry-picked this PR to the wp/7.1 branch to get it included in the next release: b750117