Deduplicate assistant replay after resume snapshots

[cbusillo]

Summary

• dedupe replayed assistant messages against any existing assistant text, including cells restored from history snapshots
• add a focused regression for snapshot restore followed by replay of the same assistant response item

Validation

• cargo test --manifest-path code-rs/Cargo.toml -p code-tui replay_item_skips_assistant_message_restored_from_snapshot --features test-helpers
• ./build-fast.sh

Refs #343