test(tui): cover dynamic remote GPT model discovery

[cbusillo]

Summary

• add regression coverage for remote GPT models that do not have local presets yet
• assert backend priority can make a newly discovered remote GPT model the single default
• assert matching remote metadata replaces a local preset instead of duplicating it

Validation

• cargo test -p code-tui remote_gpt_models_are_discovered_and_prioritized_without_local_presets --features test-helpers
• ./build-fast.sh
• JetBrains inspection closeout, scope changed_files: clean