feat(ai): support custom reasoning fields - #38227

Summary

• add optional model compatibility.reasoningField with suggested OpenAI-compatible field names and arbitrary string support
• normalize permissive Models.dev and V1 interleaved values into the compatibility object while retaining native handling for booleans
• parse custom OpenAI Chat reasoning fields and replay reasoning under the configured field on outbound assistant messages
• keep structured reasoning_details on the existing automatic metadata path and reject collisions with required assistant message fields
• document the native V2 config and regenerate the current Promise client with open-string suggestions

Testing

• bun test test/provider/openai-chat.test.ts and bun typecheck in packages/ai
• replayed the existing OpenRouter and Vercel reasoning cassettes with explicit compatibility, including signed tool-loop continuations
• targeted config, Models.dev, and model-resolution tests plus bun typecheck in packages/core
• bun test test/model.test.ts and bun typecheck in packages/schema
• bun test test/generate.test.ts and bun typecheck in packages/httpapi-codegen
• bun test and bun typecheck in packages/client and packages/plugin
• typechecks in packages/protocol, packages/server, packages/sdk-next, and packages/tui
• bun validate and bun broken-links in packages/docs