feat: expose acp thinking variants

[validatedev]

What does this PR do?

Fixes #8973 by exposing thinking variants in ACP model selection:

• Appends model+variant entries (e.g., claude-sonnet-4/high) to models.availableModels
• Parses variant from session/set_model selections and keeps session variant in sync
• Returns proper variant metadata via _meta (modelId, variant, availableVariants) instead of empty object
• Updated @agentclientprotocol/sdk to 0.13.0

How did you verify your code works?

• bun run typecheck
• Zed exposes thinking variants via model selector
• Verified session/set_model responses include correct _meta.opencode with variant info

Screenshot

[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]

The following comment was made by an LLM, it may be inaccurate:

No duplicate PRs found

[rekram1-node]

/review

[rekram1-node]

thanks!

[validatedev]

@rekram1-node ready for your re-review!

[validatedev]

@rekram1-node btw been using and there's no stability problem whatsoever! i have found an another bug on opencode acp though, gonna fix it after this merged