feat: deprecation warning (#5825)

<img width="955" height="311" alt="Screenshot 2025-10-28 at 14 26 25"
src="https://github.com/user-attachments/assets/99729b3d-3bc9-4503-aab3-8dc919220ab4"
/>

codex-rs/core/tests/suite/mod.rs

@@ -10,6 +10,7 @@ mod cli_stream;
 mod client;
 mod compact;
 mod compact_resume_fork;
+mod deprecation_notice;
 mod exec;
 mod fork_conversation;
 mod grep_files;