[fix-myself] Show latest message in resume picker

[mzeng-openai]

Summary:

• Use the latest user message when building resume picker previews.
• Update resume preview test expectations for the latest message.

Tests:

• cargo test -p codex-tui resume_picker

Tickets:

• Fixes Better distinguish between sessions in codex resume --all or codex resume #9347

❤️ from Codex

[etraut-openai]

If we make this change in the CLI, I think we should also do it for app server clients (IDE extension) for consistency. Otherwise the list of session names will differ between the two. For users who use both types of clients, it will be confusing if the session names differ.