feat: add binary codec encode/decode tests for MPT transactions

Tests cover:
- Field name encoding/decoding for all 7 new MPT fields (Hash192,
  AccountID, UInt8, UInt64, Blob types)
- Transaction type code resolution (codes 54-57)
- Ledger entry type code resolution (codes 126-127)
- Field instance metadata validation
- Full encode() roundtrip for all 4 MPT transaction types with
  hex pattern verification
- encode_for_signing() with signing prefix
- Flag serialization (combined TfMPTCanTransfer | TfMPTCanLock)
- Deterministic encoding output

Author: e-desouza