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

codex-rs/app-server-protocol/schema/json/ServerNotification.json

@@ -1618,7 +1618,7 @@
             "phase": {
               "anyOf": [
                 {
-                  "$ref": "#/definitions/MessagePhase2"
+                  "$ref": "#/definitions/MessagePhase"
                 },
                 {
                   "type": "null"
@@ -4660,13 +4660,6 @@
       "type": "string"
     },
     "MessagePhase": {
-      "enum": [
-        "commentary",
-        "finalAnswer"
-      ],
-      "type": "string"
-    },
-    "MessagePhase2": {
       "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": [
         {
@@ -5732,7 +5725,7 @@
             "phase": {
               "anyOf": [
                 {
-                  "$ref": "#/definitions/MessagePhase2"
+                  "$ref": "#/definitions/MessagePhase"
                 },
                 {
                   "type": "null"
@@ -7923,7 +7916,7 @@
             "phase": {
               "anyOf": [
                 {
-                  "$ref": "#/definitions/MessagePhase2"
+                  "$ref": "#/definitions/MessagePhase"
                 },
                 {
                   "type": "null"