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/core/tests/suite/image_rollout.rs

@@ -158,6 +158,7 @@ async fn copy_paste_local_image_persists_rollout_request_shape() -> anyhow::Resu
             },
         ],
         end_turn: None,
+        phase: None,
     };
 
     assert_eq!(actual, expected);
@@ -239,6 +240,7 @@ async fn drag_drop_image_persists_rollout_request_shape() -> anyhow::Result<()>
             },
         ],
         end_turn: None,
+        phase: None,
     };
 
     assert_eq!(actual, expected);