test(nas-backup): enable incremental in active-checkpoint test

After defaulting nas.backup.incremental.enabled to false (per the
PR-stated opt-in promise, addressed in earlier review-fix commit),
decideChainReturnsFullWhenVmHasNoActiveCheckpoint started failing
the Mockito strict-stubbing check — the new master-switch gate
short-circuits decideChain before vm.getState() and the
VM_ACTIVE_CHECKPOINT_ID lookup are reached, leaving three stubs
flagged as "unnecessary".

Mirror the same ReflectionTestUtils.setField pattern that
decideChainReturnsFullWhenIncrementalDisabled uses, but with
value="true" so we get past the master-switch gate and actually
exercise the active-checkpoint-id branch this test is meant to
cover.

Author: jmsperu

plugins/backup/nas/src/test/java/org/apache/cloudstack/backup/NASBackupProviderTest.java

@@ -411,6 +411,15 @@ public void decideChainReturnsFullWhenVmHasNoActiveCheckpoint() {
         Mockito.when(vm.getDataCenterId()).thenReturn(zoneId);
         Mockito.when(vm.getState()).thenReturn(VMInstanceVO.State.Running);
 
+        // Master switch defaults to false (opt-in by zone) — explicitly enable it for this
+        // test so we exercise the "no active_checkpoint_id" branch rather than short-circuit
+        // at the master-switch gate.
+        ReflectionTestUtils.setField(nasBackupProvider, "NASBackupIncrementalEnabled",
+                new org.apache.cloudstack.framework.config.ConfigKey<>("Advanced", Boolean.class,
+                        "nas.backup.incremental.enabled", "true",
+                        "test override — enabled", true,
+                        org.apache.cloudstack.framework.config.ConfigKey.Scope.Zone));
+
         Mockito.when(vmInstanceDetailsDao.findDetail(vmId, NASBackupChainKeys.VM_ACTIVE_CHECKPOINT_ID)).thenReturn(null);
 
         NASBackupProvider.ChainDecision decision = nasBackupProvider.decideChain(vm);