This PR adds a thinking toggle for controlling whether to use thinking
for a model in the Zed provider:

<img width="645" height="142" alt="Screenshot 2026-01-22 at 12 34 01 PM"
src="https://github.com/user-attachments/assets/9aa543fe-e708-4840-8b38-1a6fbcb78388"
/>

Previously we would create separate "Thinking" variants of the models
that supported thinking in the model selector.

This only applies to Anthropic models in the Zed provider, currently.

This is gated behind the `cloud-thinking-toggle` feature flag.

Release Notes:

- N/A

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Co-authored-by: Neel <neel@zed.dev>