RTC: Inject content function from remote changes to fix save button state

[alecgeatches]

What?

Fixes the save button not consistently appearing for RTC collaborators when another user edits blocks. Replaces the content serialization approach in #76796 with a lazy content function injection on the receiving side.

Why?

When blocks change in the editor, Gutenberg passes content as a function rather than a string in useEntityBlockEditor(). This is an optimization that defers the block serialization until it's actually needed on save. In RTC, this means the content value in the CRDT doc is often stale because these lazy functions aren't serialized and synced. Since blocks is a transient edit, block changes alone don't trigger the save button for collaborators.

The original fix (PR #76796) solved this by eagerly evaluating the content function and syncing the result via setTimeout(0). However, this has some performance costs that we'd like to avoid if possible.

How?

Instead of eagerly recomputing content on the sending side, the fix works on the receiving side. When getPostChangesFromCRDTDoc() detects that blocks have changed from a remote peer but content hasn't, it injects a lazy content function into the returned changes:

content: ({ blocks }) => __unstableSerializeAndClean(blocks)

This uses the exact same deferred serialization pattern that Gutenberg uses for local content changes. The function-valued content edit makes the entity dirty (so the save button appears), but the serialization itself is deferred until save or code editor view. This defers serialization cost for content as the editor currently does.

Testing Instructions

1. Enable RTC via Settings -> Writing -> "Enable real-time collaboration" checkbox.
2. Open the same post in two tabs.
3. Type in one window and verify the save button appears in both.
4. Undo all changes in the first window and verify the save button disappears in the second.
5. Save from the second window and verify content is correct.
6. Verify undo/redo works correctly in both windows (no extra undo levels).

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Used for: Understanding the lazy save pattern and implementing changes

[github-actions]

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• Labels found: [Package] Core data, [Feature] Real-time Collaboration.

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[github-actions]

Warning: Type of PR label mismatch

To merge this PR, it requires exactly 1 label indicating the type of PR. Other labels are optional and not being checked here.

• Required label: Any label starting with [Type].
• Labels found: [Feature] Real-time Collaboration.

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]

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

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[t-hamano]

This PR does not target trunk, so the Backport to WP 7.0 Beta/RC label appears to be unnecessary.