fix: avoid undefined array key warning in RefundProcessor::doOnSuccess

[rv-garnier]

Problem

In RefundProcessor::doOnSuccess, the loop iterates over all payments of the order and unconditionally reads the monetico_trans_uuid key from getDetails():

foreach ($order->getPayments() as $payment) {
    if ($transactionUuid === $payment->getDetails()["monetico_trans_uuid"]) {

An order can legitimately contain payments without this key (cancelled retry payments with empty details, payments from other gateways, anonymized payments, etc.). On PHP 8+ this triggers Warning: Undefined array key "monetico_trans_uuid", which is converted to an ErrorException by Symfony's error handler — interrupting the loop before the matching Monetico payment can be found. As a result, the Sylius Payment state machine transition to refunded is never applied, even though the refund itself succeeded on Monetico's side, leaving the local payment record desynchronized.

Fix

Use the null-coalescing operator to safely read the key:

if ($transactionUuid === ($payment->getDetails()['monetico_trans_uuid'] ?? null)) {

Minimal, behavior-preserving change: payments without the key simply don't match and the loop continues.