Skip redundant AUTH when SASL authentication is already complete

[tristantarrant]

Summary

• The auth() function unconditionally sent two AUTH messages during SASL authentication
• For single-step mechanisms like PLAIN, the server completes auth on the first message, making the second redundant
• The redundant AUTH can race with the server's SaslServer cleanup, causing intermittent ELY05001: Authentication mechanism exchange received a message after authentication was already complete
• Expose the server's authDone flag as a complete field in the decodeSasl result and skip the second round-trip when auth is already done
• Multi-step mechanisms like SCRAM-SHA continue to use both rounds as before

Fixes #155

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