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/codex-api/src/sse/chat.rs

@@ -331,6 +331,7 @@ async fn append_assistant_text(
             role: "assistant".to_string(),
             content: vec![],
             end_turn: None,
+            phase: None,
         };
         *assistant_item = Some(item.clone());
         let _ = tx_event

codex-rs/codex-api/src/sse/responses.rs

@@ -429,6 +429,7 @@ mod tests {
     use super::*;
     use assert_matches::assert_matches;
     use bytes::Bytes;
+    use codex_protocol::models::MessagePhase;
     use codex_protocol::models::ResponseItem;
     use futures::stream;
     use pretty_assertions::assert_eq;
@@ -492,7 +493,8 @@ mod tests {
             "item": {
                 "type": "message",
                 "role": "assistant",
-                "content": [{"type": "output_text", "text": "Hello"}]
+                "content": [{"type": "output_text", "text": "Hello"}],
+                "phase": "commentary"
             }
         })
         .to_string();
@@ -523,8 +525,11 @@ mod tests {
 
         assert_matches!(
             &events[0],
-            Ok(ResponseEvent::OutputItemDone(ResponseItem::Message { role, .. }))
-                if role == "assistant"
+            Ok(ResponseEvent::OutputItemDone(ResponseItem::Message {
+                role,
+                phase: Some(MessagePhase::Commentary),
+                ..
+            })) if role == "assistant"
         );
 
         assert_matches!(