## Summary

The authenticated ChatGPT model catalog (`/backend-api/codex/models`)
can take ~30s to respond for some accounts. Zed aborted the request
after 5s, fell back to the built-in model list, and account-specific
models disappeared from the model selector.

Just raising the timeout would make credential loading and sign-in block
for that long, since both await model discovery. Instead, they still
wait at most 5s, but the request keeps running in the background for up
to 60s and replaces the available models once it completes.

Closes #64173

## Testing

- Added `test_slow_model_catalog_does_not_block_initial_load`: the
initial load finishes after 5s with the built-in models, and a catalog
response arriving 30s later replaces them.
- `cargo test -p openai_subscribed`, `./script/clippy -p
openai_subscribed`
- Not tested against the live endpoint.

## Self-review

- [x] I've reviewed my diff for quality, security, reliability, and
performance.
- [ ] UI changes follow the
[checklist](https://zed.dev/docs/development/ui-checklist). (N/A, no UI
changes)
- [x] Tests cover the new or changed behavior.

Release Notes:

- Fixed ChatGPT Subscription models disappearing when the model catalog
takes longer than 5 seconds to load.