fix: update OutgoingMessageSender::send_response() to take Serialize (#2263)

This makes `send_response()` easier to work with.

codex-rs/mcp-server/src/lib.rs

@@ -18,6 +18,7 @@ use tracing_subscriber::EnvFilter;
 mod codex_tool_config;
 mod codex_tool_runner;
 mod conversation_loop;
+mod error_code;
 mod exec_approval;
 mod json_to_toml;
 pub mod mcp_protocol;