Guard that restores the previous state of a named retry policy on drop.
If the policy existed before, it is restored; if it was newly added, it is removed.
Resolve the matching retry policy for a given operation context.
Evaluates named policies in descending priority order; returns the
policy from the first rule whose predicate matches, or none.
Temporarily sets a named retry policy. When the returned guard is dropped,
the previous policy with the same name is restored (or removed if it didn’t exist).