Remove readOnly from nodeImageVersion in agent pool models to unblock rollback

The TypeSpec conversion in Azure/azure-rest-api-specs#38641 incorrectly
re-added readOnly to nodeImageVersion in agent pool models. This was
intentionally removed in Azure/azure-rest-api-specs#37229 to enable
agentpool rollback functionality.

The upstream spec fix is tracked in Azure/azure-rest-api-specs#41598 but
vendoring may take a week+. This is a temporary SDK-level workaround to
unblock the rollback feature that is already in preview for customers.

Removes readonly from nodeImageVersion in:
- AgentPool
- ManagedClusterAgentPoolProfileProperties
- ManagedClusterAgentPoolProfile

MachineProperties and Snapshot retain readonly as nodeImageVersion is
genuinely read-only on those resources.

Author: indu

src/aks-preview/azext_aks_preview/vendored_sdks/azure_mgmt_preview_aks/models/_models_py3.py

@@ -556,7 +556,6 @@ class AgentPool(ProxyResource):
         "e_tag": {"readonly": True},
         "os_disk_size_gb": {"maximum": 2048, "minimum": 0},
         "current_orchestrator_version": {"readonly": True},
-        "node_image_version": {"readonly": True},
         "provisioning_state": {"readonly": True},
     }
 
@@ -6517,7 +6516,6 @@ class ManagedClusterAgentPoolProfileProperties(_serialization.Model):
         "e_tag": {"readonly": True},
         "os_disk_size_gb": {"maximum": 2048, "minimum": 0},
         "current_orchestrator_version": {"readonly": True},
-        "node_image_version": {"readonly": True},
         "provisioning_state": {"readonly": True},
     }
 
@@ -7173,7 +7171,6 @@ class ManagedClusterAgentPoolProfile(ManagedClusterAgentPoolProfileProperties):
         "e_tag": {"readonly": True},
         "os_disk_size_gb": {"maximum": 2048, "minimum": 0},
         "current_orchestrator_version": {"readonly": True},
-        "node_image_version": {"readonly": True},
         "provisioning_state": {"readonly": True},
         "name": {"required": True, "pattern": r"^[a-z][a-z0-9]{0,11}$"},
     }