Fix two issues with reasoning support in the Copilot provider:

- Responses API path: use the user's thinking_effort setting instead of
hardcoding Medium effort
- Chat Completions path: compute and pass thinking_budget when thinking
is enabled, instead of unconditionally setting it to None

Self-Review Checklist:

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

Closes #52140

Release Notes:

- Fixed a bug where copilot wouldn't use the thinking level the user's
have set

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Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>
Co-authored-by: Bennet Bo Fenner <bennet@zed.dev>