A request whose egress secret cannot be added returns 503 instead of sending the placeholder

When a sandbox's server has not heard from Runtime's API within the last minute (for example, in the first seconds after a server update), it holds secret values back. Previously, an HTTPS connection opened during that time went to the destination without the secret, and every request on that connection carried the placeholder for as long as the connection stayed open, even after the server heard from the API again. Connections to a secret's host are now always handled by the step that adds the secret. A request that carries the placeholder while its value is held back returns 503 Service Unavailable with X-Runtime-Egress: secret-not-current and Retry-After: 1, and the connection closes; the value is added again from the first request after the server hears from the API. Server updates also no longer hold secret values back.