The rlpx ping command mishandled disconnect responses on two counts:
the error return from rlp.DecodeBytes was ignored, so decode failures
silently produced an "invalid disconnect message" error with no context;
and the decoder assumed the spec-compliant list form exclusively, while
older geth and some other implementations send the reason as a bare
byte.
Accept both wire forms (matching the legacy-tolerant behavior already
in p2p.decodeDisconnectMessage), and on decode failure include the raw
payload so operators can see exactly what the peer sent. Add a unit
test for the decoder covering both forms plus the empty-payload error
path.
