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/user_shell_command.rs

@@ -63,6 +63,7 @@ pub fn user_shell_command_record_item(
             text: format_user_shell_command_record(command, exec_output, turn_context),
         }],
         end_turn: None,
+        phase: None,
     }
 }