Add object_id to WP_REST_Meta_Fields::prepare_value_for_response()

[rilwis]

The method WP_REST_Meta_Fields::prepare_value_for_response() accepts 3 parameters: $value, $request and $args. It doesn't include the info about the current item (post or term) that being retrieved data. This makes it impossible to get the value for the current term if we use prepare_callback:

<?php
protected function prepare_value_for_response( $value, $request, $args ) {
        if ( ! empty( $args['prepare_callback'] ) ) {
                $value = call_user_func( $args['prepare_callback'], $value, $request, $args );
        }

        return $value;
}

So, if you register a meta for terms like this:

<?php
$args = [
        'type'         => 'string',
        'single'       => true,
        'show_in_rest' => [
                'prepare_callback' => 'my_callback_function',
        ],
];

register_meta( 'term', 'my_meta', $args );

Then inside my_callback_function, you can't get the value for the current term, especially when you request the data via /wp-json/v2/wp/categories for example.

This issue doesn't happen for posts, since WordPress sets the global $post object, so we can get it with get_post(). But there are no equivalents for terms.

Solution: I'd suggest setting the object_id inside $args to pass to the prepare_value_for_response. This way, developers can get the current term and thus, can get the correct data.

Trac Ticket: Core-62855

## Summary

• Currently, the WP_REST_Meta_Fields::prepare_value_for_response() method accepts three parameters: $value, $request, and $args. However, it does not include information about the current item (e.g., post or term) being retrieved, which leads to issues when preparing data for terms. Specifically, when using the prepare_callback function, it's impossible to access the current term's data, particularly when making requests like /wp-json/v2/wp/categories.

• This issue does not occur for posts, as WordPress automatically sets the global $post object, allowing functions like get_post() to retrieve the current post. However, there is no equivalent for terms, which causes the gap in data handling for terms.

## Solution

• This PR addresses the issue by setting the object_id (the term ID, for example) inside $args and passing it to prepare_value_for_response(). This allows developers to access the current term (or post) within the callback, ensuring that the correct data is returned, especially for taxonomies like categories.

## Changes

• Added object_id to $args in the call to prepare_value_for_response().

• Updated all relevant function calls to pass object_id.

## Why

• Fixes the issue where term data is inaccessible in callbacks.

• Provides a consistent way to access the current term, similar to how post data is handled.

• Ensures accurate data retrieval when querying for terms (e.g., categories).