JweDirectDecryptorBlock.Decrypt: unchecked _thales_aad length header reachable with forged tag → index-out-of-range / slice-bounds panic and unbounded allocation

`JweDirectDecryptorBlock.Decrypt` read a 64-bit plaintext length from the `_thales_aad` protected-header field and used it without any validation. A truncated field panics `binary.BigEndian.Uint64`, an oversized value drives an unbounded allocation, and a value larger than the decrypted block panics with a slice-bounds error.