Fix /retry to replay the last user turn

[voladoradepapantla-netizen]

Summary

• Add a domain helper that rebuilds a ChatRequest for the most recent user turn.
• Make /retry and conversation retry replay that request instead of sending an empty event.
• Print a retry title before the spinner starts.

Testing

• Added unit coverage for the retry request helper.
• Added a parser test for /retry.
• Could not run Cargo in this environment because the Rust toolchain is not installed here.

Closes #389

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