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/endpoint/chat.rs

@@ -194,6 +194,7 @@ impl Stream for AggregatedStream {
                                 text: std::mem::take(&mut this.cumulative),
                             }],
                             end_turn: None,
+                            phase: None,
                         };
                         this.pending
                             .push_back(ResponseEvent::OutputItemDone(aggregated_message));