Expose multi-agent versions in model/list (#37433)

## What changed

- Add nullable `multiAgentVersion` metadata to v2 `model/list` responses.
- Represent supported values as `disabled`, `v1`, or `v2` in the Rust API and generated JSON Schema and TypeScript exports.
- Forward the value from model catalog presets through app-server responses.

## Testing

- Extend the remote model catalog test fixture with a `v2` multi-agent version.

GitOrigin-RevId: bd3b2bbe0a3b562b4d1f03df7da2a73a35713978

codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.schemas.json

@@ -13590,6 +13590,17 @@
               "null"
             ]
           },
+          "multiAgentVersion": {
+            "anyOf": [
+              {
+                "$ref": "#/definitions/v2/MultiAgentVersion"
+              },
+              {
+                "type": "null"
+              }
+            ],
+            "description": "Multi-agent runtime declared by this model, when available."
+          },
           "serviceTiers": {
             "default": [],
             "items": {
@@ -13924,6 +13935,15 @@
           }
         ]
       },
+      "MultiAgentVersion": {
+        "description": "Multi-agent runtime supported by a model.",
+        "enum": [
+          "disabled",
+          "v1",
+          "v2"
+        ],
+        "type": "string"
+      },
       "NetworkAccess": {
         "enum": [
           "restricted",