Show accurate rollback warning when only nodeOSUpgradeChannel is active

Previously the rollback warning treated upgradeChannel and
nodeOSUpgradeChannel the same, telling users rollback 'will not succeed'
when either was enabled. This is misleading when only
nodeOSUpgradeChannel is active — the orchestrator version rollback will
succeed fine since nodeOSUpgradeChannel only manages node OS images.

Split the warning into two cases:
- upgradeChannel enabled: keep the existing hard warning
- Only nodeOSUpgradeChannel enabled: inform users the orchestrator
  rollback will proceed but the node image may be re-upgraded by
  the node OS auto-upgrade channel

Author: InduSridhar

src/aks-preview/azext_aks_preview/custom.py

@@ -2298,14 +2298,22 @@ def aks_agentpool_rollback(cmd,   # pylint: disable=unused-argument
                 "unmanaged",
             ]
 
-            if upgrade_channel_enabled or node_os_channel_enabled:
+            if upgrade_channel_enabled:
                 logger.warning(
                     "Auto-upgrade is enabled on cluster '%s' (upgradeChannel=%s, nodeOSUpgradeChannel=%s). "
                     "Rollback will not succeed until auto-upgrade is disabled. Please disable auto-upgrade to roll back the node pool.",
                     cluster_name,
                     upgrade_channel or "none",
                     node_os_upgrade_channel or "Unmanaged",
                 )
+            elif node_os_channel_enabled:
+                logger.warning(
+                    "nodeOSUpgradeChannel is enabled on cluster '%s' (nodeOSUpgradeChannel=%s). "
+                    "The orchestrator version rollback will proceed, but the node image may be "
+                    "re-upgraded automatically by the node OS auto-upgrade channel.",
+                    cluster_name,
+                    node_os_upgrade_channel,
+                )
         except Exception as ex:  # pylint: disable=broad-except
             logger.debug("Unable to retrieve auto-upgrade configuration before rollback: %s", ex)