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/thread_manager.rs

@@ -462,6 +462,7 @@ mod tests {
                 text: text.to_string(),
             }],
             end_turn: None,
+            phase: None,
         }
     }
     fn assistant_msg(text: &str) -> ResponseItem {
@@ -472,6 +473,7 @@ mod tests {
                 text: text.to_string(),
             }],
             end_turn: None,
+            phase: None,
         }
     }