A permanent failure reported after that generation ended (the client
connected again from the same endpoint) ended the generation that replaced
it and said goodbye to the new client. The callback now acts only while the
generation that sent the datagram is live, which is the one in `sessions`.
That makes two guards unreachable, so they are deleted: `peer.unsendable`
before the goodbye (the first permanent failure ends the generation, so a
second callback from it is already ignored) and the conditional delete (a
live generation is the entry at its key). A refusal's goodbye is sent
best-effort, as the transport and the document server send theirs, instead
of through the failure handling meant for datagrams a session owes.
Tests: late failure after a reconnect, for the transport's generation check
and for the operation server.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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