Initial-response-only SASL authenticators do not complete exchanges

[OskarEichler]

Problem

A custom SASL authenticator that supplies an initial response and has no further challenge response can remain incomplete because the exchange does not mark that valid one-shot path done.

Prepared atomic commit d60b07dc5b8276ee0141ec0410227d05d5c284e1 completes initial-response-only exchanges while retaining multi-step and cancellation behavior. Dual-Ruby exchange models and the full suite pass; package and Rails checks pass. No repository tests were modified.

[nevans]

@OskarEichler Please proofread your bot's issues and PRs before submitting them. If you wouldn't submit it as a human, you shouldn't let your bots do it on your behalf.

What I'm specifically referring to here is this: "Prepared atomic commit ....". If you've prepared an atomic commit, where is that commit?! This should've been a PR, not an issue (or at least an issue with a linked PR)!

Or "No repository tests were modified." Of course no tests were modified... because this is an issue, not a PR!