CVE-2026-59640: OpenPGP CFB quick-check oracle active on symmetric/session-key paths

Affects Bouncy Castle for Java LTS before 2.73.12. PGPSymmetricKeyEncryptedData.getDataStream() still performs the CFB quick check on the repeated IV bytes and throws before the MDC is verified, on the symmetric and session-key paths. The public-key path already omits this check to avoid the Mister-Zuccherato oracle, and no property gates the remaining one. An attacker with adaptive chosen-ciphertext access can distinguish the early failure from later ones and recover two plaintext bytes per roughly 2^15 queries against SEIPDv1 messages.