Fix N+1 in cluster agents resolver

What does this MR do and why?

This spec was quarantined a while ago as it was failing intermittently (see #350868 (comment 817969674) for initial investigation).

Removing the quarantine revealed that there was indeed an N+1 loading user_detail and user_preference records. I didn't find a definitive reason for this to be failing only some of the time, but with the new preloads added I cannot replicate the failure.

#350868 (closed)

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