Support for the `uniqueItems` keyword was added in [48357], but wasn't included in the list of permitted keywords for `WP_REST_Controller::get_endpoint_args_for_item_schema()`. This prevented the keyword from being automatically supported if specified in a controller's item schema.
Props TimothyBlynJacobs.
Merges [48796] to the 5.5 branch.
Fixes#51021.
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