Make RSA/ElGamal PKCS#1 v1.5 decryption constant time

[michalz-zoom]

KBPGP RSA decryption code suffers from padding oracle attacks. There are two major issues:

• Decrypting a message fails with distinct errors in the "unpadding" function: "Failed to find expected header", "didn't get 0x00 separator" etc.
• Early exits in the unpadding code. This creates a timing side channel.

This PR hardens the OpenPGP message decryption path starting at Message::_decrypt in processor.iced. PCKS padding and session key recovery now avoids early exits, distinct errors or branching.

• eme_pkcs1_decode scans the whole decrypted encoded message instead of stopping at the separator.
• Padding failures are returned as a validity bit, not distinct public errors.
• RSA/ElGamal callers (decrypt_and_unpad) do not abort on padding failure.
• import_key_pgp_ct avoids branching on the session-key algorithm/checksum result by checking all supported ciphers and selecting a key.
• The OpenPGP data packet is still decrypted with either the real session key or a random fallback key.
• Final failure is collapsed to "Unable to decrypt".