refactor(openai-compatible): deprecate normalized chat option shims

[giulio-leone]

Summary

• split OpenAI-compatible chat options into:
  • stable normalized options (user)
  • legacy normalized compatibility options (reasoningEffort, textVerbosity, strictJsonSchema)
• keep legacy normalized options functional for backwards compatibility
• emit deprecation warnings when legacy normalized options are used
• continue passing provider-native options through providerOptions.<providerName>
• update OpenAI-compatible provider docs to clarify preferred provider-native configuration

Why

Issue #12461 highlights that OpenAI-compatible is an abstraction layer, so hard-coding provider-specific option names in the normalized model options is brittle. This change keeps compatibility while moving toward a cleaner contract.

Details

• Added openaiCompatibleLanguageModelChatLegacyOptions for deprecated normalized option shims.
• Kept openaiCompatibleLanguageModelChatOptions focused on stable user.
• Updated request-arg building to:
  • parse both stable and legacy options from openai-compatible, openaiCompatible, and provider-name keys
  • warn when legacy normalized options are used
  • filter both stable and legacy keys from passthrough request fields to avoid duplicates
• Updated docs section for OpenAI-compatible chat options and deprecated legacy options.

Tests

• pnpm --filter @ai-sdk/openai-compatible exec vitest --config vitest.node.config.js --run src/chat/openai-compatible-chat-language-model.test.ts
• pnpm --filter @ai-sdk/openai-compatible exec eslint src/chat/openai-compatible-chat-options.ts src/chat/openai-compatible-chat-language-model.ts src/chat/openai-compatible-chat-language-model.test.ts
• pnpm --filter @ai-sdk/openai-compatible type-check
• pnpm exec prettier --check $(git diff --name-only)

Closes #12461

Attribution request

If this pull request is merged, I would be grateful if contributor credit could be included in the related changelog or release notes for implementing this fix (@g97iulio1609).