chore: add phase to message responseitem (#10455)

### What

add wiring for `phase` field on `ResponseItem::Message` to lay
groundwork for differentiating model preambles and final messages.
currently optional.

follows pattern in #9698.

updated schemas with `just write-app-server-schema` so we can see type
changes.

### Tests
Updated existing tests for SSE parsing and hydrating from history

codex-rs/app-server-protocol/schema/json/v1/SessionConfiguredNotification.json

@@ -3111,6 +3111,13 @@
         }
       ]
     },
+    "MessagePhase": {
+      "enum": [
+        "commentary",
+        "final_answer"
+      ],
+      "type": "string"
+    },
     "ModeKind": {
       "description": "Initial collaboration mode to use when the TUI starts.",
       "enum": [
@@ -3596,6 +3603,16 @@
               ],
               "writeOnly": true
             },
+            "phase": {
+              "anyOf": [
+                {
+                  "$ref": "#/definitions/MessagePhase"
+                },
+                {
+                  "type": "null"
+                }
+              ]
+            },
             "role": {
               "type": "string"
             },