Add model availability NUX metadata (#12972)

- replace show_nux with structured availability_nux model metadata
- expose availability NUX data through the app-server model API
- update shared fixtures and tests for the new field

codex-rs/app-server-protocol/schema/json/v2/ModelListResponse.json

@@ -22,6 +22,16 @@
     },
     "Model": {
       "properties": {
+        "availabilityNux": {
+          "anyOf": [
+            {
+              "$ref": "#/definitions/ModelAvailabilityNux"
+            },
+            {
+              "type": "null"
+            }
+          ]
+        },
         "defaultReasoningEffort": {
           "$ref": "#/definitions/ReasoningEffort"
         },
@@ -92,6 +102,17 @@
       ],
       "type": "object"
     },
+    "ModelAvailabilityNux": {
+      "properties": {
+        "message": {
+          "type": "string"
+        }
+      },
+      "required": [
+        "message"
+      ],
+      "type": "object"
+    },
     "ModelUpgradeInfo": {
       "properties": {
         "migrationMarkdown": {