An outbound connection that cannot reach Runtime's web proxy returns 503 Service Unavailable with the reason

Previously the sandbox's relay closed such a connection without a response, which clients reported as Connection reset by peer. Now the response carries X-Runtime-Egress: proxy unreachable (<reason>) and Retry-After: 1. A sandbox holds about 1,000 outbound connections at the same time, fewer than limits.connections for a sandbox with more than one vCPU, until a later release raises it; past that, new connections return this response.