[Key Vault] Make ManagedHsmSkuName an extensible enum (modelAsString: true) (#44008)

* update managed hsm sku enum to string

* update

* Fix tsp formatting: wrap @typeChangedFrom decorator to satisfy TypeSpec Validation Format rule

---------

Co-authored-by: Neerja Gangwar <ngangwar@microsoft.com>

Author: neerjagangwar

specification/keyvault/resource-manager/Microsoft.KeyVault/KeyVault/models.tsp

@@ -588,7 +588,7 @@ enum KeyVaultNameUnavailableReason {
 /**
  * SKU of the managed HSM Pool
  */
-#suppress "@azure-tools/typespec-azure-core/no-enum" "FIXME: Update justification, follow aka.ms/tsp/conversion-fix for details"
+#suppress "@azure-tools/typespec-azure-core/no-enum" "Closed enum retained unchanged for already-released GA API versions 2025-05-01 and 2026-02-01 per Azure Breaking Changes Board guidance; superseded by the extensible union ManagedHsmSkuNameV2 starting in 2026-03-01-preview."
 enum ManagedHsmSkuName {
   Standard_B1,
   Custom_B32,
@@ -597,6 +597,29 @@ enum ManagedHsmSkuName {
   Custom_C10,
 }
 
+/**
+ * SKU of the managed HSM Pool
+ */
+@Versioning.added(KeyVault.Versions.v2026_03_01_preview)
+union ManagedHsmSkuNameV2 {
+  string,
+
+  /** Standard_B1 SKU */
+  Standard_B1: "Standard_B1",
+
+  /** Custom_B32 SKU */
+  Custom_B32: "Custom_B32",
+
+  /** Custom_B6 SKU */
+  Custom_B6: "Custom_B6",
+
+  /** Custom_C42 SKU */
+  Custom_C42: "Custom_C42",
+
+  /** Custom_C10 SKU */
+  Custom_C10: "Custom_C10",
+}
+
 /**
  * The parameters used to create a key.
  */
@@ -1906,7 +1929,11 @@ model ManagedHsmSku {
   /**
    * SKU of the managed HSM Pool
    */
-  name: ManagedHsmSkuName;
+  @Versioning.typeChangedFrom(
+    KeyVault.Versions.v2026_03_01_preview,
+    ManagedHsmSkuName
+  )
+  name: ManagedHsmSkuNameV2;
 }
 
 /**

specification/keyvault/resource-manager/Microsoft.KeyVault/KeyVault/preview/2026-03-01-preview/openapi.json

@@ -4611,7 +4611,7 @@
           "description": "SKU Family of the managed HSM Pool"
         },
         "name": {
-          "$ref": "#/definitions/ManagedHsmSkuName",
+          "$ref": "#/definitions/ManagedHsmSkuNameV2",
           "description": "SKU of the managed HSM Pool"
         }
       },
@@ -4642,7 +4642,7 @@
         ]
       }
     },
-    "ManagedHsmSkuName": {
+    "ManagedHsmSkuNameV2": {
       "type": "string",
       "description": "SKU of the managed HSM Pool",
       "enum": [
@@ -4653,8 +4653,35 @@
         "Custom_C10"
       ],
       "x-ms-enum": {
-        "name": "ManagedHsmSkuName",
-        "modelAsString": false
+        "name": "ManagedHsmSkuNameV2",
+        "modelAsString": true,
+        "values": [
+          {
+            "name": "Standard_B1",
+            "value": "Standard_B1",
+            "description": "Standard_B1 SKU"
+          },
+          {
+            "name": "Custom_B32",
+            "value": "Custom_B32",
+            "description": "Custom_B32 SKU"
+          },
+          {
+            "name": "Custom_B6",
+            "value": "Custom_B6",
+            "description": "Custom_B6 SKU"
+          },
+          {
+            "name": "Custom_C42",
+            "value": "Custom_C42",
+            "description": "Custom_C42 SKU"
+          },
+          {
+            "name": "Custom_C10",
+            "value": "Custom_C10",
+            "description": "Custom_C10 SKU"
+          }
+        ]
       }
     },
     "ManagedHsmTrigger": {