fix(provider): expand $ref/$defs for DeepSeek compatibility

[lexlian]

Issue for this PR

Closes #32829
Closes #29220

Type of change

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

What does this PR do?

DeepSeek API rejects JSON Schema 2020-12 $ref pointers in MCP tool schemas, causing AttributeError: 'NoneType' object has no attribute 'lookup' when using MCP servers with $defs references (Asana, Notion).

This PR adds expandRefs() to ProviderTransform.schema() that inlines $ref targets from $defs/definitions before sending schemas to DeepSeek. It handles nested $ref references, preserves override fields on referencing nodes (e.g. a description on the $ref node wins over the definition's description), and guards against circular references via a seen set.

The approach mirrors PR #12292 (closed, not merged) which proposed the same fix for Gemini.

How did you verify your code works?

• bun typecheck — pass (full repo typecheck via turbo)
• bun test test/provider/transform.test.ts — 255 pass, 0 fail (7 new tests added)

Tests added:

• Expands simple $ref with $defs
• Expands nested $ref references
• Handles circular $ref
• Preserves description when expanding $ref
• Expands definitions (legacy format)
• Does not expand $ref for non-DeepSeek providers
• Expands $ref in array items

Screenshots / recordings

N/A — non-UI change.

Checklist

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

[github-actions]

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

[justmessingaround]

same issue here. how do we get it merged?

[github-actions]

Automated PR Cleanup

Thank you for contributing to opencode.

Due to the high volume of PRs from users and AI agents, we periodically close older PRs using automated criteria so maintainers can focus review time on the most active and community-supported contributions.

This PR was closed because it matched the following cleanup criteria:

• The PR was created more than 1 month ago
• The PR had fewer than 2 positive reactions
• Positive reactions are counted as thumbs-up, heart, celebration, or rocket reactions on the PR

PRs created within the last month are not affected by this cleanup.

If you believe this PR was closed incorrectly, or if you are still actively working on it, please leave a comment explaining why it should be reopened. A maintainer can review and reopen it if appropriate.

Thanks again for taking the time to contribute.