REST API: Prevent taxonomy REST bases from overwriting post item properties

[csergozoltan]

Trac ticket: https://core.trac.wordpress.org/ticket/65855

This PR prevents taxonomies with conflicting REST bases from overwriting existing post item properties.

The existing _doing_it_wrong() warning added for #48401 is preserved, but conflicting taxonomies are excluded from the item-level REST taxonomy set so they no longer participate in:

• item schema properties;
• prepared item responses;
• term assignment;
• term assignment permission checks;
• REST action links; and
• schema links.

The conflict-free taxonomy list is initialized while building the item schema and reused by the relevant item-level paths. This avoids lazily calling get_item_schema() from the helper, which could introduce recursion in subclasses such as WP_REST_Menu_Items_Controller.

Collection-level taxonomy query behavior is intentionally unchanged.

Tests added for:

• preserving the existing type schema property when a taxonomy name conflicts;
• preserving the existing type response property;
• the WP_REST_Menu_Items_Controller subclass schema path.

Testing:

• npm run test:php -- --group 65855
  • OK (3 tests, 7 assertions)
• npm run test:php -- --filter WP_Test_REST_Posts_Controller
  • OK (283 tests, 2637 assertions)
• npm run test:php -- --filter Tests_REST_WpRestMenuItemsController
  • OK (41 tests, 441 assertions)
• PHPCS passes for all four modified files.
• git diff --check reports no whitespace errors.

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