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/tests/clients.rs

@@ -309,6 +309,7 @@ async fn streaming_client_retries_on_transport_error() -> Result<()> {
                 text: "hi".to_string(),
             }],
             end_turn: None,
+            phase: None,
         }],
         tools: Vec::<Value>::new(),
         parallel_tool_calls: false,