Every nats-server this module starts renders `auth_callout.xkey` or does
not start, and the responder serves only that server, in the same
container and generation. A request without `Nats-Server-Xkey` therefore
comes from no supported server, so the plaintext framing is gone: such a
request is logged at warn and gets no reply, and the server that sent it
times out the connect as a denial. A signed denial would have had to go
out unsealed, which is the path this removes.
Also scopes two claims to what is true: the xkey's public half is read by
the swarm-nats-auth policy only, and the metadata-delete gap in the
replace-key procedure is about the two xkey paths, not all of
`secret/metadata/`.
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