[REF] simplify interaction with objects in complete order

Per civicrm#18382 this line seems to be heavily tested so
& per the test we can use the 'already calculated value' for payment_processor_id rather than look
it up in yet another place.

This adds a query but in most cases queries can be removed before calling this function as
the various objects are not really accessed anymore (apart from the few lines just above this)

CRM/Contribute/BAO/Contribution.php

@@ -13,6 +13,7 @@
 use Civi\Api4\ContributionPage;
 use Civi\Api4\ContributionRecur;
 use Civi\Api4\Participant;
+use Civi\Api4\PaymentProcessor;
 
 /**
  *
@@ -4428,7 +4429,7 @@ public static function completeOrder($input, $ids, $objects, $isPostPaymentCreat
     // If paymentProcessor is not set then the payment_instrument_id would not be correct.
     // not clear when or if this would occur if you encounter this please fix here & add a unit test.
     if (empty($contributionParams['payment_instrument_id']) && isset($contribution->_relatedObjects['paymentProcessor']['payment_instrument_id'])) {
-      $contributionParams['payment_instrument_id'] = $contribution->_relatedObjects['paymentProcessor']['payment_instrument_id'];
+      $contributionParams['payment_instrument_id'] = PaymentProcessor::get()->addWhere('id', '=', $paymentProcessorId)->addSelect('payment_instrument_id')->execute()->first()['payment_instrument_id'];
     }
 
     if ($recurringContributionID) {