Wire PQS query knob overrides into query planning. Add applyPqsOverrides() on QueryKnobConfiguration, which iterates pre-parsed QuerySettingsKnobOverlay entries and writes them into snapshot slots. Update the DeferredFn lambda in ExpressionContext to call applyPqsOverrides() when QuerySettings has a non-empty queryKnobs field. After this change, PQS knob overrides flow from setQuerySettings through to the query planner.