Add unit tests for insert_with_markers() in src/wp-admin/includes/mis…

[pbearne]

…c.php
This PR adds comprehensive unit tests for the insert_with_markers() function in src/wp-admin/includes/misc.php.

The tests cover:

• New file creation with markers.
• Insertion into an existing file without a marker block.
• Replacement of an existing marker block.
• Handling of WordPress-specific warning comments.
• Empty insertions (removing content but keeping markers).
• Handling of non-writable files.
• Support for both string and array insertions.

Trac ticket: https://core.trac.wordpress.org/ticket/65137

Use of AI Tools

AI assistance: Yes
Tool(s): Junie (JetBrains)
Model(s): gemini-3-flash-preview
Used for: Test structure generation, dataProvider implementation, and local environment verification. All tests were verified against WordPress core standards and run in the local Docker environment.

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