BIP328: Use Base58Check in ExtendedKey.deserialize and validate length

[sashass1315]

• Replace raw Base58 decode and manual slice with base58.decode_check(xpub) to enforce checksum validation as required by BIP-32 and as implied by the method docstring.
• Add strict len(data) == 78 check before parsing to ensure the serialized extended key length matches the BIP-32 78-byte structure.
• This prevents accepting corrupted or truncated extended keys and aligns deserialization behavior with Base58Check semantics.