BIP375 requires 33-byte scan-key keydata for PSBT_GLOBAL_SP_ECDH_SHARE, PSBT_GLOBAL_SP_DLEQ, PSBT_IN_SP_ECDH_SHARE, and PSBT_IN_SP_DLEQ.
The reference structure validator currently checks the value lengths of these fields but ignores their keydata lengths. This change enforces the 33-byte keydata requirement for all four fields, with errors identifying the field and, for per-input fields, the input index.
Add unit tests covering missing, short, and oversized keydata, accepted 33-byte keydata, and the existing value-length checks. Document the unit test command in the validation reference README.