Unit Tests: Add tests for extract_from_markers() and stop empty values being returned

[pbearne]

The function extract_from_markers() in src/wp-admin/includes/misc.php lacks unit tests. This PR adds tests to ensure it correctly extracts content between markers in a file, handling missing files, different markers, and comments.

…sc.php

Reference: https://core.trac.wordpress.org/ticket/65136

This PR adds unit tests for the extract_from_markers() function in src/wp-admin/includes/misc.php.
The tests cover:

• Extraction from a standard WordPress .htaccess block.
• Handling of comments within markers.
• Multiple marker blocks.
• Non-existent files and missing markers.

Trac ticket: https://core.trac.wordpress.org/ticket/65136

## Use of AI Tools
AI assistance: Yes
Tool(s): Junie (JetBrains)
Model(s): gemini-3-flash-preview
Used for: Generating test cases, implementing set_up/tear_down for temporary file management, and validating extraction logic.

[pbearne]

In testing, I found that empty lines where beeing return in the array of values

Add code to stop this

[desrosj]

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