Add unit tests for update_recently_edited()

[pbearne]

This ticket adds unit tests for the update_recently_edited() function in wp-admin/includes/misc.php. These tests ensure that the function correctly manages the list of recently edited files in the recently_edited option, maintaining order and a maximum of 5 items.

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Description:
This PR adds unit tests for the update_recently_edited() function in wp-admin/includes/misc.php. These tests ensure that the function correctly manages the list of recently edited files in the recently_edited option.

The tests cover:

• Adding a new file to an empty list.
• Adding a new file to an existing list (maintaining order).
• Ensuring a maximum of 5 files are kept in the list.
• Moving an existing file to the top of the list when it's edited again.
• Handling duplicate entries.

Trac ticket: https://core.trac.wordpress.org/ticket/65174

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.

[westonruter]

@pbearne I think it would be preferable to instead open these PRs as part of the overall #64894 ticket. This will reduce the amount of work for you. You wouldn't have to open any of these tickets: #65170, #65171, #65172, #65173, #65174, #65175, #65176, #65178, #65179, #65180, #65181, #65182, #65183, #65184, #65185, #65186, #65187, #65192, #65193, #65194, #65195, #65196, #65197, #65198, #65199, #65200, #65201, #65202, #65203, #65225, #65226, #65227, #65228, #65236, #65237.

@desrosj Is this a blessed use of that catchall ticket?