[FEATURE]: Add a reasoning variant for Mistral Small

[petit-blaireau]

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Mistral Small has got a thinking mode now (reasoningEffort:"high").
https://docs.mistral.ai/capabilities/reasoning/adjustable
It would be nice if it were possible to toggle the thinking variant directly from OpenCode.
Today OpenCode doesn't offer it because variants(model) always returns {} for mistral models.

[github-actions]

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This issue might be a duplicate of existing issues. Please check:

• missing reasoning variants for multiple providers #12155: Missing reasoning variants for multiple providers — same root cause where variants(model) returns {} for non-supported providers
• GLM models excluded from reasoning variants — no way to toggle thinking at runtime #18598: GLM models excluded from reasoning variants — identical issue pattern (variants() returns {}, preventing runtime thinking toggle)

If your request is specifically scoped to Mistral Small and isn't covered by the above, feel free to clarify the distinction.

[petit-blaireau]

This is not a duplicate of those issues. This is about adding proper support for a new Mistral Small model (well maybe not so new now 😄)

For it to work with the Mistral provider, an upgrade to @ai-sdk/mistral might be required in order to have support for reasoningEffort. Other providers should already be fine though.