feat: use OAI Responses API MessagePhase type directly in App Server v2

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

@@ -374,11 +374,23 @@
       "type": "string"
     },
     "MessagePhase": {
-      "enum": [
-        "commentary",
-        "finalAnswer"
-      ],
-      "type": "string"
+      "description": "Classifies an assistant message as interim commentary or final answer text.\n\nProviders do not emit this consistently, so callers must treat `None` as \"phase unknown\" and keep compatibility behavior for legacy models.",
+      "oneOf": [
+        {
+          "description": "Mid-turn assistant text (for example preamble/progress narration).\n\nAdditional tool calls or assistant output may follow before turn completion.",
+          "enum": [
+            "commentary"
+          ],
+          "type": "string"
+        },
+        {
+          "description": "The assistant's terminal answer text for the current turn.",
+          "enum": [
+            "final_answer"
+          ],
+          "type": "string"
+        }
+      ]
     },
     "PatchApplyStatus": {
       "enum": [