Wire up openai_to_anthropic in the /v1/messages handler: buffer
upstream OpenAI response, parse, translate to Anthropic format
(stop_reason mapping, usage field names, content blocks).
5 integration tests covering round-trip translation, system prompt,
content blocks, and error cases.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
