test: close diagnostic dispatch gate before signaling

tests/MBN_STOCK_WEBVIEW.Playout.Tests/PgmConnectDiagnosticTests.cs

@@ -400,13 +400,16 @@ public sealed class PgmConnectDiagnosticTests
 
         public bool TryPreventKtapConnect()
         {
-            PreventionAttempted.Set();
             var previous = Interlocked.CompareExchange(ref _dispatchGate, 2, 0);
             if (previous == 1 && LastKtapConnectState == PlayoutKtapConnectState.NotAttempted)
             {
                 LastKtapConnectState = PlayoutKtapConnectState.Attempted;
             }
 
+            // Publish the test synchronization point only after the dispatch gate
+            // is closed. Signalling first lets the blocked safety probe resume and
+            // race KTAP dispatch before this prevention attempt has taken effect.
+            PreventionAttempted.Set();
             return previous == 0;
         }