fix(provider): normalize openai-compatible schema required field

[ca0fgh]

Issue for this PR

Closes #15540

Type of change

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

What does this PR do?

Some OpenAI-compatible providers (for example DeepSeek via compatible gateways) can reject payloads when any object schema has required: null.

This PR normalizes outgoing POST schema payloads for @ai-sdk/openai, @ai-sdk/openai-compatible, and @ai-sdk/azure: when schema type is object and required is not an array, it sets required to [].

It keeps existing OpenAI item-id stripping behavior intact.

How did you verify your code works?

• Added regression tests in test/provider/openai-schema-normalization.test.ts.
• Ran:
  • bun test --timeout 30000 test/provider/openai-schema-normalization.test.ts
  • bun test --timeout 30000 test/provider/provider.test.ts test/provider/openai-schema-normalization.test.ts
• Reproduced the real DeepSeek failure before fix and verified it no longer returns at '/required': got null, want array after this patch.

Screenshots / recordings

N/A (non-UI change)

Checklist

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

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[github-actions]

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