RTC: saving reverts other collaborators' unsaved edits (template, title, and other synced fields) #79935

Description

When one collaborator saves, other collaborators' unsaved edits to synced fields are silently reverted: the save pushes the full server response into the shared CRDT document, rebroadcasting stale database values for fields the saver didn't change. Last-write-wins scalars (template, status, slug, sticky, format, taxonomies) snap back immediately; title/content/excerpt can revert through the rich-text merge. Hidden when the saver's payload already carries the peer's edit (fast local sync); surfaces when a save races the edit sync (slower transports, autosaves, programmatic saves). First seen as a lost template selection during RTC testing. Related, not a duplicate: #76402 (opposite direction — unsaved edits persisting).

Step-by-step reproduction instructions

Needs wp_collaboration_enabled, two users, and a page (Twenty Twenty-Five's "Page No Title" template is pages-only).

1. Both users open the same page; confirm 2 online.
2. B: change the template to "Page No Title", don't save.
3. A: save without B's edit, e.g. in the console: wp.data.dispatch( 'core' ).saveEntityRecord( 'postType', wp.data.select( 'core/editor' ).getCurrentPostType(), { id: wp.data.select( 'core/editor' ).getCurrentPostId(), content: '<!-- wp:paragraph --><p>test</p><!-- /wp:paragraph -->' } ) (Save draft on a fast local site may carry B's edit and hide the bug).
4. B's template reverts within a few seconds. Same with an unsaved title edit.

Environment info

• Gutenberg trunk, wp-env, Twenty Twenty-Five (block theme), no other plugins.