AesGcmCryptor.Open passes attacker-controlled nonce to cipher.AEAD.Open without length check, causing panic on wrong-size IV

`AesGcmCryptor.Open` passed a caller-supplied nonce straight to `cipher.AEAD.Open` without checking its length. The Go standard library **panics** rather than returning an error when a GCM nonce is not exactly `NonceSize()` bytes, so a JWE carrying a wrong-sized IV crashes the decrypting goroutine.