assistant2: Add testing environment variables (#27789)

To make it easier to design UIs for some of these scenarios. This PR
adds specifically two variables:
- `ZED_SIMULATE_NO_THREAD_HISTORY`
- `ZED_SIMULATE_NO_LLM_PROVIDER`

Release Notes:

- N/A

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Co-authored-by: Agus Zubiaga <hi@aguz.me>
d50905e000Danilo Leal committed on 3/31/2025, 6:19:42 PM· committed by GitHubparent0729d24
2 files changedLine totals unavailable