- Restrict fallback_models to only reference preset names in model_presets.
- Add schema validation to reject unknown preset names in fallback_models.
- Remove build_provider_for_model() since bare model fallback is no longer supported.
- Simplify make_provider_factory() to only look up presets by name.
- Update onboard UI to remove "Add custom model" option from fallback chain.
- Update tests to use preset names instead of bare model strings in fallback chains.
- Fix test imports referencing deleted _make_provider function.
