Unit Tests: Add tests for insert_with_markers()

[pbearne]

The function insert_with_markers() in src/wp-admin/includes/misc.php lacks unit tests. This PR adds tests to ensure it correctly inserts or replaces content between markers in a file, handles file creation, permissions, and respects existing content outside markers.

…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.

[desrosj]

Adding the tests focus, which is used to indicate a ticket is solely focused on adding tests.