Real-time collaboration: Fix persisted document meta and add tests

[chriszarate]

What?

Fix the inspection of persisted CRDT documents to correctly detect changes. Add type safety and tests to prevent recurrence.

Why?

• Bug 1 accidentally nested CRDT document meta in a meta property. As a result, persisted CRDT documents were not correctly identified as being from persistence, resulting in incorrect merge behavior and CRDT document invalidation.
• Bug 2 incorrectly merged some "floating dates," which meant that some persisted CRDT documents were incorrectly invalidated.

How?

• Provide the CRDT document meta correctly.
  • Introduce a type that will prevent this bug in the future.
• Do not merge "floating dates."
• Add tests to cover the functions in question.

[github-actions]

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Co-authored-by: chriszarate <czarate@git.wordpress.org>
Co-authored-by: maxschmeling <maxschmeling@git.wordpress.org>

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