Whether an HTTP status from the initial modern POST should trigger the
legacy HTTP+SSE (2024-11-05) backward-compatibility fallback. Per
basic/transports §Backwards Compatibility, a client probing a single modern
endpoint should fall back when the POST fails with 400 Bad Request, 404 Not
Found, or 405 Method Not Allowed.
Whether an HTTP status from the initial modern POST should trigger the legacy HTTP+SSE (2024-11-05) backward-compatibility fallback. Per basic/transports §Backwards Compatibility, a client probing a single modern endpoint should fall back when the POST fails with 400 Bad Request, 404 Not Found, or 405 Method Not Allowed.