fix(repl): allow shift-tab mode toggle

[andreabergia]

Summary

• allow () to toggle between Python and Assistant modes
• keep behavior unchanged
• add test coverage for toggling back with

Testing

• cargo fmt --all --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --features test-support test_support_harness_renders_and_toggles_mode -- --nocapture
• cargo test