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/src/tasks/mod.rs

@@ -265,6 +265,7 @@ impl Session {
                     ),
                 }],
                 end_turn: None,
+                phase: None,
             };
             self.record_into_history(std::slice::from_ref(&marker), task.turn_context.as_ref())
                 .await;