test(scheduler): Lambda no-refresh tests pass vacuously since #932 #962

Two scheduler tests no longer test anything since #932: TestScheduler_ListRecommendations_LambdaSkipsBackgroundRefresh (internal/scheduler/scheduler_test.go:1060) and TestScheduler_ListRecommendations_ColdStart_Lambda_NeverMarksOrCollects (:1118).

Why

#932 made ListRecommendations kick a cold-start or stale-cache collection only when the context carries WithBackgroundCollection, via mayCollect := BackgroundCollectionAllowed(ctx) (scheduler.go:~1153). These two tests still call it with a plain context.Background(). Nothing is ever collected, whether or not the Lambda guard exists.

Evidence

During the independent review of #960, the !s.isLambda condition at scheduler.go:~1183 was removed in a scratch copy, and both tests still passed. The same review did not probe the isLambda guards at scheduler.go:~1452 and ~1502.

#960 fixed the same problem for the stale-refresh tests that #938 added.

Fix

• Run both tests with WithBackgroundCollection(context.Background()), so the only thing preventing collection is the Lambda guard.
• Confirm each test fails when its isLambda guard is removed.
• Check the guards at ~1452 and ~1502 for the same gap.

Test-only. No production behaviour change.