Reject changing the member of running time entries

Moving a running time entry to another member could give that member's
user a second running time entry. Stopping the time entry and changing
the member in the same request is still allowed.

app/Exceptions/Api/TimeEntryMemberCanNotBeChangedWhileRunningApiException.php

@@ -0,0 +1,15 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Exceptions\Api;
+
+class TimeEntryMemberCanNotBeChangedWhileRunningApiException extends ApiException
+{
+    public const string KEY = 'time_entry_member_can_not_be_changed_while_running';
+
+    public function report(): bool
+    {
+        return true;
+    }
+}