More outbound connections, and a clear answer at the limit

A paid sandbox now holds 512 outbound connections at once, or 128 per vCPU up to 1,024, and opens 100 new ones a second or more. Past its limit, a new connection gets 503 with X-Runtime-Egress: too many open connections and Retry-After: 1 instead of a reset, and a connection silent for a minute is closed first to make room. See the sandbox environment.