fixeshyperhive/hyperhive#2839. pr_assign_reviewer's own doc-comment
claimed full idempotency (already-requested = no-op), but conflated
'still pending' with 'already reviewed' - Forgejo clears a fulfilled
reviewer from requested_reviewers, so re-requesting them isn't a
no-op, it dismisses the standing review. now checks latest_reviews
for a non-superseded review from the target user first and skips
the request instead of blindly posting it.
