REST API: Restore global post state in WP_REST_Posts_Controller.Ensur…

[liaisontw]

This PR addresses a long-standing issue in WP_REST_Posts_Controller::prepare_item_for_response() where the global $post object was not restored after calling setup_postdata().

When the REST API processes post data, it temporarily modifies the global $post state to ensure core functions (like get_the_content()) work correctly. However, failing to reset this state leads to "global context pollution," especially when the REST API is called internally during a custom WP_Query loop or other custom development contexts.

Changes included in this PR:

Manual State Backup: Saves the original $GLOBALS['post'] before modification.

Robust Restoration: Ensures the global $post is restored to its original state (including null) in both the main execution path and the early-return path for HEAD requests.

Comprehensive Unit Tests: 1. Verified that the global $post is restored to its previous object after the API call.
2. Verified that the global $post is restored to null if it wasn't set before the call.

Trac ticket: https://core.trac.wordpress.org/ticket/43502

Use of AI Tools
AI assistance: Yes
Tool(s): Gemini
Model(s): Gemini 3 Flash
Used for: Identifying the core architectural issue (global state pollution vs. wp_reset_postdata limitations) and drafting the technical explanation for the PR description. The final implementation was verified against the latest Trac patch (#43502.6.diff) and local PHPUnit tests.

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[apermo]

Other than the indentation on the phpdoc, looks good to me