Request timeouts on sign-up and webhook routes

A request to a route you call before you have a key (the MCP sign-up, GitHub's webhooks and install link, and an agent's reports) whose body has not all arrived after 5 seconds is now answered 408 request_timeout straight away: send it again. Before, it waited 15 seconds or more and was answered 503 api_unavailable as if Runtime had failed. See errors.