fix: update hash format validation to -01 spec (plain base64url, no prefix)

Go ValidateHashFormat was still validating the old -00 format
(algorithm:base64url with sha-256/sha-384/sha-512 prefix). Updated to
validate plain base64url without prefix per -01 spec and RFC 9449.
Python was already updated but uncommitted. Both refimpls now match.

refimpl/go-lang/ect/errors.go

@@ -23,5 +23,5 @@ var (
 	ErrInvalidJTI          = errors.New("ect: jti must be UUID format")
 	ErrInvalidWID          = errors.New("ect: wid must be UUID format when set")
 	ErrPredLength          = errors.New("ect: pred exceeds max length")
-	ErrHashFormat          = errors.New("ect: inp_hash/out_hash must be algorithm:base64url (e.g. sha-256:...)")
+	ErrHashFormat          = errors.New("ect: inp_hash/out_hash must be plain base64url (no prefix)")
 )