## Context

We were seeing lots of pending title generation, which should only
happen if we don't have a summarization model.

`handle_models_updated_event` unconditionally overwrote the thread's
summarization model on every registry event, even with `None`. 

We should only setting if explicitly changed by the user or we haven't
set it yet, and only if we actually have one.

It is hard to reproduce this issue.. but I don't think this code was
right in the first place anyway.


## Self-Review Checklist

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- N/A