Real-time collaboration: Don't sync core/freeform block without content attribute

[chriszarate]

What?

Real-time collaboration: Don't sync core/freeform (Classic) block without a defined content attribute.

Why?

A core/freeform block without a defined content attribute triggers an open modal. If we sync the block in this "not-ready" state, connected peers will experience an open modal as well.

How?

Withhold syncing until the content attribute is defined.

Testing Instructions

1. Check out branch.
2. Add a "Classic" block.
3. Ensure modal only opens for the user who added the block.

Screenshots or screencast

Before

classic-before.mov

After

classic-after.mov

[github-actions]

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

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[github-actions]

Flaky tests detected in 802e899.
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/18857912632
📝 Reported issues:

• [Flaky Test] should refresh the page when stylesheet loading fails #71722 in /test/e2e/specs/interactivity/router-styles.spec.ts