missing tests for src/wp-admin/includes/bookmark.php functions

None of the ten functions in src/wp-admin/includes/bookmark.php had direct test coverage; wp_insert_link() and wp_delete_link() appeared only as fixtures for the get_bookmark() tests.

This adds a test file per function under tests/phpunit/tests/admin/includes/bookmark/, mirroring the source path and the structure used for wp-admin/includes/misc.php in [62369]. The cases pin the documented contracts: the empty-URL and name-fallback returns in wp_insert_link(), its $wp_error branch, the manage_links check and $_POST escaping in edit_link(), category replacement and integer casting in wp_set_link_cats(), the partial-merge behaviour of wp_update_link(), and the branches of wp_link_manager_disabled_message().

No production code is changed.

Trac ticket: https://core.trac.wordpress.org/ticket/66019

## Use of AI Tools
AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 5
Used for: writing the test cases and checking that each one fails when the function under test is broken. All changes were reviewed and validated by me.

Missing tests for bookmark.php

Trac ticket: https://core.trac.wordpress.org/ticket/66019

AI Disclosure:

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

[pbearne]

Let's use @khokansardar patch