[vibe-d/mongodb] [critical] [mongo8] No client session support

[gedaiu]

We parse logicalSessionTimeoutMinutes from the handshake but never actually create or use sessions. Without them we can't do transactions, causal consistency, or retryable writes.

Need a startSession() on MongoClient, attach lsid to commands, and clean up with endSession(). Also need implicit sessions for operations that require them.

Blocks transactions and retryable writes.