check_fd_agreement is the guard that keeps a descriptor and the request
it arrived with in agreement, and both of its rejections were untested.
An fd-taking op with no descriptor must not fall back to anything: a
temp file or the response socket would send an agent's state somewhere
the caller never asked for. The mirror case matters for a different
reason -- returning the error is what drops the OwnedFd and closes it,
so ignoring a stray descriptor instead would leak one per bad request
in a long-lived root process.
The third test pins both agreeing combinations, so the check is
rejecting mismatches rather than descriptors in general. Descriptors
are real /dev/null handles so the closing drop is genuinely exercised.
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