Create a message in Anthropic's format, directly against OpenRouter

Proxies to OpenRouter's Anthropic-compatible /messages. Request and response are Anthropic-shaped, not OpenAI-shaped: system is a top-level field, max_tokens is required (defaulted to 4096 here), and the reply is a type: "message" envelope with a content block array. With stream: true the response is an Anthropic event stream — named SSE events (message_start, content_block_delta, message_delta, message_stop) and no [DONE] sentinel — so the Anthropic SDK can be pointed at this route unchanged. OpenRouter extras (provider, route, transforms) are forwarded, under the same price cap as the other endpoints; models (OpenRouter's own fallback-routing list) and plugins (paid addons like web search) are stripped and silently ignored — neither is validated against this proxy's catalog/policy/ price cap or billed for. Billing normalizes Anthropic's usage block, whose input_tokens excludes cache reads and writes. Unlike the OpenAI-shaped routes the openhuman envelope is opt-in (openhuman.include_metadata: true), so a strict Anthropic client is never handed an event or field it does not expect.