fix: remove VMSS node-vm-size blocker and add VMs pool test

Address review comments:

1. Remove the InvalidArgumentValueError in update_auto_scaler_properties()
   that blocked --node-vm-size for VMSS pools. This check was added when
   --node-vm-size only supported VirtualMachines pools, but now VMSS
   pools support VM size resize via rolling upgrade.

2. Add test case for VirtualMachines pool to verify update_vm_size() is
   a no-op (VMs pools handle VM size via the autoscaler update path).

3. Add HISTORY.rst entry for the new feature.

Author: Wen Huang

src/aks-preview/HISTORY.rst

@@ -11,6 +11,7 @@ To release a new version, please select a new version number (usually plus 1 to
 
 Pending
 +++++++
+* `az aks nodepool update`: Support `--node-vm-size` to resize VM size of an existing VMSS-based agent pool (preview). Requires AFEC registration `Microsoft.ContainerService/AgentPoolVMSSResize`.
 
 19.0.0b28
 +++++++

src/aks-preview/azext_aks_preview/agentpool_decorator.py

@@ -1967,13 +1967,6 @@ def update_auto_scaler_properties(self, agentpool: AgentPool) -> AgentPool:
         if self.context.get_vm_set_type() == CONST_VIRTUAL_MACHINES:
             return agentpool
 
-        vm_size = self.context.raw_param.get("node_vm_size")
-        if vm_size is not None:
-            raise InvalidArgumentValueError(
-                "--node-vm-size can only be used with virtual machines agentpools. "
-                "Updating VM size is not supported for virtual machine scale set agentpools."
-            )
-
         (
             update_cluster_autoscaler,
             enable_cluster_autoscaler,

src/aks-preview/azext_aks_preview/tests/latest/test_agentpool_decorator.py

@@ -2691,6 +2691,28 @@ def common_update_vm_size(self):
         )
         self.assertEqual(dec_agentpool_2, ground_truth_agentpool_2)
 
+        # Test case 3: VirtualMachines pool with node_vm_size provided (should be no-op)
+        dec_3 = AKSPreviewAgentPoolUpdateDecorator(
+            self.cmd,
+            self.client,
+            {"node_vm_size": "Standard_D4s_v3"},
+            self.resource_type,
+            self.agentpool_decorator_mode,
+        )
+        agentpool_3 = self.create_initialized_agentpool_instance(
+            vm_size="Standard_D2s_v3"
+        )
+        # Set pool type to VirtualMachines - use the correct attribute based on decorator mode
+        from azure.cli.command_modules.acs._consts import AgentPoolDecoratorMode
+        if self.agentpool_decorator_mode == AgentPoolDecoratorMode.MANAGED_CLUSTER:
+            agentpool_3.type = CONST_VIRTUAL_MACHINES
+        else:
+            agentpool_3.type_properties_type = CONST_VIRTUAL_MACHINES
+        dec_3.context.attach_agentpool(agentpool_3)
+        dec_agentpool_3 = dec_3.update_vm_size(agentpool_3)
+        # vm_size should remain unchanged for VMs pools
+        self.assertEqual(dec_agentpool_3.vm_size, "Standard_D2s_v3")
+
     def common_update_upgrade_strategy(self):
         # Test case 1: No upgrade strategy provided (should not change agentpool)
         dec_1 = AKSPreviewAgentPoolUpdateDecorator(