Suggest mode 6: Bump payload schema to v2 (foundations)

[adamsilverstein]

What

Bumps the suggestion payload schemaVersion from 1 to 2 and tightens parseSuggestionPayload to:

• Refuse payloads written by a newer editor (schemaVersion > SCHEMA_VERSION) — surfaces an explicit "newer editor" notice instead of silently dropping unknown op types.
• Migrate older payloads forward (schemaVersion < SCHEMA_VERSION) — pure version-field stamp; v1 payloads only ever carried attribute-set ops and remain valid as v2 payloads.
• Treat a missing schemaVersion field as v1 (only the v1 implementation could have emitted it).

This is plumbing for the structural-suggestion stack tracked in #77434. No user-visible change.

Why

Without the bump, a v1 reader receiving a v2 payload with block-insert-after / block-remove / block-move ops would silently apply only the attribute-set ops and drop the rest — a confusing partial-apply. Refusing the payload outright hands the user a clear "this suggestion was made by a newer editor" notice and only the Reject affordance.

The shape is otherwise unchanged: v1 payloads carry only attribute-set ops, which are still valid in v2. The migration step in parseSuggestionPayload is a no-op rewrite — it just stamps the version field forward.

Testing the complete feature

To exercise the complete Suggest-mode feature (structural suggestions + inline preview marks) end-to-end, check out the combined integration branch try/suggest-mode-combined, which merges this stack (#77967 → #77979) with the inline-preview stack (#77869) on top of add-suggestion-mode:

git fetch origin try/suggest-mode-combined && git checkout try/suggest-mode-combined
npm install && npm run build
npm run wp-env start

The combined branch is testing-only — review still happens on the individual PRs.

Testing instructions

Then in wp-admin:

1. Open a post for editing.
2. Open the editor's "Options" kebab → switch the editor intent to Suggest.
3. block-remove suggestion: click "Remove block" on a paragraph from the toolbar.
   • The block stays in place with strikethrough + 50% opacity.
   • The Notes sidebar shows a "Remove block: paragraph" thread with Apply / Reject buttons.
   • Apply → block removes for real, marker clears, comment marked applied.
   • Reject → marker clears, block stays.
4. block-insert-after suggestion: insert a new block via the inserter while in Suggest mode.
   • The block lands at the chosen position with reduced opacity + dashed green outline.
   • Sidebar shows an "Insert block: " thread with Apply / Reject.
   • Apply → marker clears, block stays as a normal part of the post.
   • Reject → block removes.
5. block-move suggestion: drag a paragraph to a new position (or use "Move up" / "Move down").
   • The block sits at its new position with reduced opacity + dotted green outline.
   • Sidebar shows a "Move block: " thread with Apply / Reject and a from→to descriptor.
   • Apply → marker clears, block stays at the new position.
   • Reject → block moves back to its original position.

For the multi-peer scenario, open the same post in a second browser tab as a different user. Suggestions auto-saved on tab A appear in tab B's sidebar; Apply/Reject on tab B propagates back through the sync layer without the originating tab's interceptor reverting the apply (covered by isAcceptedSuggestionChange for attribute-set today; structural multi-peer accept hardening tracked as a follow-up).

Stack

1. This PR (#77967) — Schema v2 bump (foundations)
2. #77968 — Capture block-remove (mechanism)
3. #77970 — Render block-remove preview (UI)
4. #77971 — Capture block-insert-after (mechanism)
5. #77973 — Render block-insert-after preview (UI)
6. #77978 — Capture block-move (mechanism)
7. #77979 — Render block-move preview (UI)

Refs #77434, #73411.

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

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

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[github-actions]

Size Change: +134 B (0%)

Total Size: 7.51 MB

📦 View Changed

Filename	Size	Change
build/scripts/editor/index.min.js	482 kB	+134 B (+0.03%)

_{compressed-size-action}

[github-actions]

Flaky tests detected in f6c4e6f.
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/25390203164
📝 Reported issues:

• [Flaky Test] post title and content are editable and blocks can be inserted #77720 in /test/e2e/specs/editor/various/post-content-focus-mode.spec.js
• [Flaky Test] should show correct homepage actions based on current homepage or posts page #77385 in /test/e2e/specs/site-editor/homepage-settings.spec.js