Add unit tests for heartbeat_autosave() in wp-admin/includes/misc.php

[pbearne]

Description:
This PR adds unit tests for the heartbeat_autosave() function in wp-admin/includes/misc.php. These tests ensure that the function correctly processes Heartbeat data to trigger post autosaves, and returns the appropriate response messages for success, error, and empty states.

The tests cover:

• Verification that the response remains unchanged if no wp_autosave data is provided.
• Successful autosave with correct success message and date format.
• Handling of WP_Error returned by wp_autosave().
• Handling of empty results (e.g., when content/title are empty).

Trac ticket: https://core.trac.wordpress.org/ticket/65201

AI Disclosure:

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

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