fix(provider): extract OpenAI-shaped error messages

[AayushMainali-Github]

Issue for this PR

Closes#36410

Type of change

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

What does this PR do?

Provider error parsing used body.message || body.error || body.error?.message. For OpenAI-shaped bodies, body.error is an object, so the real error.message was skipped and users got a generic status line or raw JSON.

Extraction now prefers string message / error / error.message fields. Added unit tests for the OpenAI object shape and string error.

How did you verify your code works?

Ran bun test test/provider/error.test.ts in packages/opencode.

Checklist

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

[github-actions]

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