feat(core): add top-level thinking configuration to language model calls

[giulio-leone]

Summary

• add a top-level thinking field to LanguageModelV3CallOptions and AI SDK CallSettings
• forward thinking through generateText, streamText, generateObject, streamObject, and ToolLoopAgent
• map top-level thinking to provider-specific reasoning controls in OpenAI chat, OpenAI responses, OpenAI-compatible chat, and Anthropic
• keep provider-specific options backward compatible while giving precedence to top-level thinking
• document the new option in core references and provider docs

Details

• Added LanguageModelV3Thinking type:
  • { type: 'enabled', effort?: 'low' | 'medium' | 'high', budgetTokens?: number }
  • { type: 'disabled' }
• Added runtime validation for thinking in prepareCallSettings.
• Added telemetry hardening for object-valued call settings (serializes plain objects).
• Added warnings when thinking.budgetTokens is used with OpenAI/OpenAI-compatible where unsupported.

Tests

• pnpm --filter ai exec vitest --config vitest.node.config.js --run src/prompt/prepare-call-settings.test.ts src/middleware/default-settings-middleware.test.ts src/agent/tool-loop-agent.test.ts src/generate-text/generate-text.test.ts src/generate-text/stream-text.test.ts src/generate-object/generate-object.test.ts src/generate-object/stream-object.test.ts
• pnpm --filter @ai-sdk/openai exec vitest --config vitest.node.config.js --run src/chat/openai-chat-language-model.test.ts src/responses/openai-responses-language-model.test.ts
• 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/anthropic exec vitest --config vitest.node.config.js --run src/anthropic-messages-language-model.test.ts
• pnpm --filter ai type-check
• pnpm --filter @ai-sdk/provider type-check
• pnpm --filter @ai-sdk/openai type-check
• pnpm --filter @ai-sdk/openai-compatible type-check
• pnpm --filter @ai-sdk/anthropic type-check
• package-level eslint checks on all touched source/test files

Closes #12516

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).