Fix ReplicaSet unit test label to match updated test data

Update test_replica_set_build() to use 'backend-rs' label instead of
'backend', matching the label change made to prevent resource adoption
conflicts with DeploymentTest.

This completes the label isolation work - the unit test was setting
the label to 'backend' but asserting it should be 'backend-rs'.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: cuppett

tests/ReplicaSetTest.php

@@ -20,7 +20,7 @@ public function test_replica_set_build()
 
         $rs = $this->cluster->replicaSet()
             ->setName('mariadb-rs')
-            ->setLabels(['tier' => 'backend'])
+            ->setLabels(['tier' => 'backend-rs'])
             ->setAnnotations(['mariadb/annotation' => 'yes'])
             ->setReplicas(3)
             ->setTemplate($pod);