SetActivityWorkoutNotFound is a per-user-scoped store method but never got
the adversarial cross-user coverage CLAUDE.md's testing conventions require
for such features. Extend the existing isolation test with a mismatched-user
call (must no-op) and a correct-user call (must exclude the activity from
that user's own ActivitiesMissingWorkout/count without touching the other
user's pending activity).