Show final assistant replay after stale snapshot

[cbusillo]

Summary

• Replays assistant messages from raw history items after restoring a rich snapshot when the snapshot does not already contain that assistant text.
• Adds a resume replay regression for the case where a snapshot ends at tool output but the persisted conversation contains a later final assistant closeout.

Fixes #317.

Validation

• cargo test -p code-tui --test resume_replay
• ./build-fast.sh