REST API: Clarify the invalid stored value error messages - #14840

Trac ticket: https://core.trac.wordpress.org/ticket/64019

What

Rewords the rest_invalid_stored_value error messages so they explain the actual cause. The error code and 500 status are unchanged for back-compat.

All three messages share one mechanism: when a stored value fails schema validation, the REST API returns it as null. A client that echoes the response back (as the block editor does) sends that null, and the update is refused so the stored data is not deleted.

• Single meta (WP_REST_Meta_Fields::update_value()): "The %s meta field cannot be reset to null because its stored value does not match its registered schema."
• Non-single meta with null items: "The %s meta field cannot be updated because the request contains null values. This usually means one or more of its stored values do not match its registered schema."
• Settings (WP_REST_Settings_Controller::update_item()): "The %s setting cannot be reset to null because its stored value does not match its registered schema."

Tests

The two meta field code paths had no coverage for rest_invalid_stored_value. Added tests for both, including the echo-back case, asserting the stored values are left untouched.

Testing instructions

1. Register an integer meta: register_post_meta( 'post', 'my_int', array( 'type' => 'integer', 'single' => true, 'show_in_rest' => true ) );
2. Store an invalid value directly: update_post_meta( $post_id, 'my_int', 'not-a-number' );
3. GET /wp/v2/posts/<id> returns "my_int": null.
4. POST /wp/v2/posts/<id> with {"meta":{"my_int":null}} returns 500 with the new message, and the stored value is unchanged.

Reproduced and verified on trunk with WordPress Playground (nightly, PHP 8.3).