Waking a sandbox on a busy server waits for room instead of failing

When the server that holds a paused sandbox has no room for it, POST /v1/sandboxes/{id}:wake now pauses Runtime's own lower-priority work on that server, as automatic wakes already did, and wakes the sandbox once the room is free, for up to 20 seconds. Previously the request returned 503 no_capacity immediately. A wake that still finds no room returns 503 no_capacity as before. Memory, processes and files are kept either way.