This PR updates the BIP-375 reference validator to enforce the serialized length of PSBT_OUT_SP_V0_LABEL.
BIP-375 defines the field value as a 32-bit little-endian unsigned integer, so PSBT_OUT_SP_V0_LABEL must contain exactly 4 bytes. Previously, the validator only checked that PSBT_OUT_SP_V0_LABEL requires PSBT_OUT_SP_V0_INFO, allowing malformed values such as a one-byte label when valid SP_V0_INFO was present.