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/review.rs

@@ -530,6 +530,7 @@ async fn review_input_isolated_from_parent_history() {
                 text: "parent: earlier user message".to_string(),
             }],
             end_turn: None,
+            phase: None,
         };
         let user_json = serde_json::to_value(&user).unwrap();
         let user_line = serde_json::json!({
@@ -549,6 +550,7 @@ async fn review_input_isolated_from_parent_history() {
                 text: "parent: assistant reply".to_string(),
             }],
             end_turn: None,
+            phase: None,
         };
         let assistant_json = serde_json::to_value(&assistant).unwrap();
         let assistant_line = serde_json::json!({