fix(xai): default store to false for Responses

[geraint0923]

Match OpenAI-family stateless Responses defaults so @ai-sdk/xai injects include=["reasoning.encrypted_content"] for multi-turn reasoning rehydration.

Issue for this PR

Closes #36631

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

OpenAI-family Responses clients already default to store: false so multi-turn reasoning can work without server-side stored responses. xAI uses the Responses API via @ai-sdk/xai, but OpenCode never set store: false for it.

In @ai-sdk/xai, when store === false, the Responses language model automatically adds:

include: ["reasoning.encrypted_content"]

Without that, encrypted reasoning is not requested, so follow-up turns can’t get the encrypted state needed for rehydration.

This PR adds @ai-sdk/xai to the same default path as OpenAI / Copilot / Mantle in ProviderTransform.options() and smallOptions(), so xAI models get store: false by default (still with the existing promptCacheKey behavior).

Note: this guarantees we request encrypted reasoning on the response. Full next-turn replay also depends on @ai-sdk/xai input conversion preserving that content; that path still has gaps and is not fully fixed here.

How did you verify your code works?

• Unit tests in packages/opencode/test/provider/transform.test.ts for xAI defaults (store: false, including custom provider IDs using @ai-sdk/xai)
• Full transform.test.ts suite: 295 pass
• Broader test/provider/ + test/session/llm.test.ts: 454 pass
• Prettier + oxlint on touched files (0 errors)
• Pre-push typecheck via turbo: 30 packages pass

Screenshots / recordings

If this is a UI change, please include a screenshot or recording.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

If you do not follow this template your PR will be automatically rejected.

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍