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/client_websockets.rs

@@ -211,6 +211,7 @@ fn message_item(text: &str) -> ResponseItem {
         role: "user".into(),
         content: vec![ContentItem::InputText { text: text.into() }],
         end_turn: None,
+        phase: None,
     }
 }