Tests: Add unit tests for wp_doc_link_parse()

[pbearne]

This ticket adds comprehensive unit tests for the wp_doc_link_parse() function in wp-admin/includes/misc.php to ensure it correctly identifies function calls while ignoring local definitions and class methods.

Description:
This PR adds unit tests for the wp_doc_link_parse() function in wp-admin/includes/misc.php. These tests ensure that the function correctly parses PHP content to identify function calls while correctly ignoring local function definitions and class method calls.

The tests cover:

• Empty and invalid inputs.
• Simple and multiple function calls.
• Duplicate and sorted function names.
• Ignoring local function definitions and object methods.
• Correct handling of static class methods.
• Verification of the documentation_ignore_functions filter.

Trac ticket: https://core.trac.wordpress.org/ticket/65182

AI Disclosure:

• AI assistance: Yes
• Tool(s): Junie (JetBrains)
• Model(s): gemini-3-flash-preview
• Used for: Code analysis, test implementation, and workflow management.

[desrosj]

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

[SergeyBiryukov]

In 62369:

Tests: Add unit tests for wp_doc_link_parse().

Follow-up to [10607].

Props pbearne.
Fixes #65182.