Editor: Add admin notice for Gutenberg block markup in content

[Infinite-Null]

https://core.trac.wordpress.org/ticket/65335

Summary

Adds a warning notice in the Classic Editor when the post content contains Gutenberg block markup (<!-- wp:). Since block markup is stored inside HTML comments, the Visual tab may appear empty even though content still exists.

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Changes

• Added a conditional check in edit-form-advanced.php that detects WordPress block syntax (<!-- wp:) in $post->post_content
• Displays a dismissible warning admin notice using wp_admin_notice() when block markup is found
• Notice is placed immediately before the <form> element so it is visible without interrupting the editing flow

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Before / After

Before	After

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Testing Instructions

1. Create or open a post that contains Gutenberg block markup in its content:

<!-- wp:group {"layout":{"type":"constrained"}} -->
<div class="wp-block-group"> </div>
<!-- /wp:group -->

3. Open the post in the Classic Editor.
4. Verify that the warning notice appears above the editor form.
5. Confirm the notice is dismissible.

[github-actions]

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[github-actions]

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Props ankitkumarshah.

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