Store API: Fix payment method validation (fixes COD) (#9375)

* Widen enum for validation before cart is loaded

* Improve error message on checkout

src/StoreApi/Routes/V1/Checkout.php

@@ -617,12 +617,14 @@ private function get_request_payment_method( \WP_REST_Request $request ) {
 		}
 
 		if ( ! isset( $available_gateways[ $request_payment_method ] ) ) {
+			$all_payment_gateways = WC()->payment_gateways->payment_gateways();
+			$gateway_title        = isset( $all_payment_gateways[ $request_payment_method ] ) ? $all_payment_gateways[ $request_payment_method ]->get_title() : $request_payment_method;
 			throw new RouteException(
 				'woocommerce_rest_checkout_payment_method_disabled',
 				sprintf(
 					// Translators: %s Payment method ID.
-					__( 'The %s payment gateway is not available.', 'woo-gutenberg-products-block' ),
-					esc_html( $request_payment_method )
+					__( '%s is not available for this order—please choose a different payment method', 'woo-gutenberg-products-block' ),
+					esc_html( $gateway_title )
 				),
 				400
 			);

src/StoreApi/Schemas/V1/CheckoutSchema.php

@@ -119,7 +119,9 @@ public function get_properties() {
 				'description' => __( 'The ID of the payment method being used to process the payment.', 'woo-gutenberg-products-block' ),
 				'type'        => 'string',
 				'context'     => [ 'view', 'edit' ],
-				'enum'        => array_values( wp_list_pluck( WC()->payment_gateways->get_available_payment_gateways(), 'id' ) ),
+				// Validation may be based on cart contents which is not available here; this returns all enabled
+				// gateways. Further validation occurs during the request.
+				'enum'        => array_values( WC()->payment_gateways->get_payment_gateway_ids() ),
 			],
 			'create_account'    => [
 				'description' => __( 'Whether to create a new user account as part of order processing.', 'woo-gutenberg-products-block' ),