transitionParticipants() - Improve $contactDetails cache maintenance

This revises a very recent commit that touches `static $contactDetails` cache.

Imagine you have two calls to `transitionParticipants()` which involve different-but-overlapping sets:

* First invocation involves Alice and Bob
* Second invocation involves Bob and Carol

The first invocation loads Alice and Bob into `$contactDetails`.  The second
invocation identifies Carol as missing. We need to load Carol. Here's the change:

* Before: Carol overwrites Bob in `$contactDetails`. Bob goes missing.
* After: Alice, Bob, and Carol all exist in `$contactDetails`.

CRM/Event/BAO/Participant.php

@@ -1236,7 +1236,7 @@ public static function transitionParticipants(
 
     //get all required contacts detail.
     if (!empty($contactIds)) {
-      $contactDetails = civicrm_api3('Contact', 'get', ['id' => ['IN' => $contactIds, 'return' => 'display_name']])['values'];
+      $contactDetails += civicrm_api3('Contact', 'get', ['id' => ['IN' => $contactIds, 'return' => 'display_name']])['values'];
     }
 
     //get all required events detail.