[TEST ONLY] RTC: Discard stale compactions when a newer one already exists

[ramonjd]

Note

I don't have any context on RTC and this was AI generated. I think it might be better to unblock folks by skipping the test, and wait for the experts to chime in. Skip PR here: #77991

Follow up to #77980

When a compaction arrives but a newer compaction already exists after the client's cursor, the server was storing the stale data as a regular update, but the newer compaction already subsumes that state. Discard it silently instead.

Also consolidates to a single Gutenberg_WP_HTTP_Polling_Sync_Server implementation (the established override pattern for shipping fixes ahead of Core), removing the duplicate WP_HTTP_Polling_Sync_Server class that was never loaded when Core's version exists.

What?

Closes

Why?

How?

Testing Instructions

Testing Instructions for Keyboard

Screenshots or screencast

Before	After

Use of AI Tools

A lot

[ramonjd]

Gutenberg-specific class exists so that tests/other functionality works in the plugin in the absence of Core's WP_HTTP_Polling_Sync_Server

[ramonjd]

Closing. #77986 exists. I didn't find it!

[github-actions]

Flaky tests detected in c450e9f.
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/25412398194
📝 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 insert media, convert to blocks, and undo in one step #77705 in /test/e2e/specs/editor/blocks/classic.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