fix(mac): default AltTester server port for Unity 6 builds

MacBuilderUnity6 only set AltServerPort when ALTSERVER_PORT is provided.
In CI we don't set it, which can prevent the AltTester server from listening
on the expected port (13000) and causes macOS UI tests to fail to connect.

Author: rodrigo-fournier-immutable

sample/Assets/Editor/MacBuilderUnity6.cs

@@ -159,6 +159,13 @@ private static void SetupAltTester(string[] scenes)
         {
             instrumentationSettings.AltServerPort = int.Parse(port);
         }
+        else
+        {
+            // Keep parity with Unity 2021 builder + Windows Unity 6 builder.
+            // CI does not set ALTSERVER_PORT, and without a default the AltTester server
+            // may not start on the expected port (13000).
+            instrumentationSettings.AltServerPort = 13000;
+        }
 
         instrumentationSettings.ResetConnectionData = true;
         AltBuilder.InsertAltInScene(scenes[0], instrumentationSettings);