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/app-server/tests/suite/list_resume.rs

@@ -308,6 +308,7 @@ async fn test_list_and_resume_conversations() -> Result<()> {
             text: fork_history_text.to_string(),
         }],
         end_turn: None,
+        phase: None,
     }];
     let resume_with_history_req_id = mcp
         .send_resume_conversation_request(ResumeConversationParams {