Sandboxes: creates no longer return 501 or an immediate 503 for some accounts

POST /v1/sandboxes could return 501 (for example env_unavailable or feature_unavailable) or an immediate 503 for request bodies the API supports. A create rejected before anything was applied now succeeds in the same request, and a retry with the same Idempotency-Key returns the same sandbox. A 503 busy whose create may have been applied is still returned, so a retry never creates a second sandbox.