doRefund: send required description, fire alterPaymentProcessorParams, use APIv4 for currency lookup

- Mollie's create-refund API marks 'description' as required; without one
  the refund is rejected. Send the caller's description when provided,
  otherwise a sensible default. The description may be shown to the
  customer (e.g. on their bank statement) depending on the payment method.
- Fire the existing alterPaymentProcessorParams hook before sending, with
  an 'action' => 'Refund' marker (removed again after the hook), so
  extensions can customise the refund parameters/description the same way
  they already can for purchases.
- Resolve the fallback currency via APIv4 FinancialTrxn::get instead of
  raw SQL, per the CiviCRM coding standards preference for API access.

Verified live against Mollie (iDeal): refund accepted and re_... id
returned as refund_trxn_id.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: Richard van Oosterhout

CRM/Core/Payment/OmnipayMultiProcessor.php

@@ -318,21 +318,31 @@ public function doRefund(&$params) {
     if (empty($currency)) {
       // Look up the currency of the original transaction so the refund is
       // issued in the same currency; fall back to the default currency.
-      $currency = CRM_Core_DAO::singleValueQuery(
-        'SELECT currency FROM civicrm_financial_trxn WHERE trxn_id = %1',
-        [1 => [$params['trxn_id'], 'String']]
-      ) ?: $this->getCurrency($params);
+      $originalTrxn = \Civi\Api4\FinancialTrxn::get(FALSE)
+        ->addSelect('currency')
+        ->addWhere('trxn_id', '=', $params['trxn_id'])
+        ->execute()
+        ->first();
+      $currency = $originalTrxn['currency'] ?? $this->getCurrency($params);
     }
     $this->ensurePaymentProcessorTypeIsSet();
     $this->createGatewayObject();
     $this->setProcessorFields();
 
+    $refundOptions = [
+      'action' => 'Refund',
+      'transactionReference' => $params['trxn_id'],
+      'amount' => \Civi::format()->machineMoney($params['amount'], $currency),
+      'currency' => $currency,
+      // Mollie requires a description; it may be shown to the customer
+      // (e.g. on their bank statement) depending on the payment method.
+      'description' => $params['description'] ?? ts('Refund of payment %1', [1 => $params['trxn_id']]),
+    ];
+    CRM_Utils_Hook::alterPaymentProcessorParams($this, $params, $refundOptions);
+    unset($refundOptions['action']);
+
     try {
-      $response = $this->gateway->refund([
-        'transactionReference' => $params['trxn_id'],
-        'amount' => \Civi::format()->machineMoney($params['amount'], $currency),
-        'currency' => $currency,
-      ])->send();
+      $response = $this->gateway->refund($refundOptions)->send();
     }
     catch (\Exception $e) {
       throw new \Civi\Payment\Exception\PaymentProcessorException('Refund failed: ' . $e->getMessage());