Fix NASBackupProviderTest failures for zone-scoped ConfigKey values

The test helper overrideConfigValue() was only setting _value on
ConfigKey, but zone-scoped configs (valueIn(zoneId)) fall back to
_defaultValue when s_depot is null in test context. Also set
_defaultValue via ReflectionTestUtils to ensure valueIn() returns
the expected test value.

Fixes: 4 assertion failures (compression, bandwidth, encryption,
integrity_check details all returned null) and 1 error
(encryption without passphrase expected CloudRuntimeException
but got NullPointerException from null config value).

Author: jmsperu

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

@@ -357,9 +357,13 @@ public void testGetVMHypervisorHostFallbackToZoneWideKVMHost() {
 
     private void overrideConfigValue(final ConfigKey configKey, final Object value) {
         try {
+            // Set _value for value() calls
             Field f = ConfigKey.class.getDeclaredField("_value");
             f.setAccessible(true);
-            f.set(configKey, value);
+            f.set(configKey, value != null ? configKey.valueOf((String) value) : null);
+
+            // Also set _defaultValue via Spring's ReflectionTestUtils (handles final fields)
+            ReflectionTestUtils.setField(configKey, "_defaultValue", String.valueOf(value));
         } catch (IllegalAccessException | NoSuchFieldException e) {
             Assert.fail(e.getMessage());
         }