message: Add tests for serde de/serialization of the `Digest`
diff: @@ -343,6 +343,21 @@ mod test {
  bytesrepr::test_serialization_roundtrip(&hash);
  }
  + #[proptest]
  + fn serde_roundtrip(data: [u8; Digest::LENGTH]) {
  + let original_hash = Digest(data);
  + let serialized = serde_json::to_string(&original_hash).unwrap();
  + let deserialized_hash: Digest = serde_json::from_str(&serialized).unwrap();
  + assert_eq!(original_hash, deserialized_hash);
  + }
  +
  + #[test]
  + fn serde_custom_serialization() {
  + let serialized = serde_json::to_string(&Digest::SENTINEL_RFOLD).unwrap();
  + let expected = format!("\"{}\"", Digest::SENTINEL_RFOLD);
  + assert_eq!(expected, serialized);
  + }
  +
  #[test]
  fn hash_known() {
  // Data of length less or equal to [ChunkWithProof::CHUNK_SIZE_BYTES]
  