## Summary
- For SIWC users, update the model list merging logic to prefer the
model list fetched from the backend over the bundled model list (this is
needed for special cases where users have a more limited set of models
they're allowed to use)
- Add or update tests covering the revised cache behavior
## Testing
- Added/updated unit tests in
`codex-rs/models-manager/src/manager_tests.rs`
- Not run (not requested)
