errors.Join drops operator hints from the gates that need them most

[lexfrei]

errors.Join in cockroachdb/errors does not carry the hints of the errors it joins, a single one included. Measured on the real path:

direct     hints=[do this]
join(1)    hints=[]
join(2)    hints=[]
hint(join) hints=[outer hint]

main.go renders operator guidance from errors.GetAllHints, so any hint attached below a join is dead. Two gates lose theirs.

runPostUpgradeVersionVerifyInner joins errors carrying postUpgradeVersionMismatchHint. That hint is the whole playbook for the A/B-rollback case: the two hypotheses, how to tell them apart, --post-upgrade-reconcile-window, --skip-post-upgrade-verify. None of it prints.

runPostApplyGates joins errors from verifyAppliedState, which are WithHint-bearing too.

Both suites stay green because they assert the hint on the unjoined helper. The fix is one line per site: errors.WithHint(joined, hint) after the join, with the assertion moved onto the joined error.