Intermittent failure: `Test_Client_InsertNotificationsAreDeduplicatedAndDebounced`

Intermittent failure in Test_Client_InsertNotificationsAreDeduplicatedAndDebounced.

Sample run:
https://github.com/riverqueue/river/actions/runs/18163358692/job/51699168994?pr=1049

cd /home/runner/work/river/river && go test ./... -race -timeout 2m
--- FAIL: Test_Client_InsertNotificationsAreDeduplicatedAndDebounced (2.80s)
    client_test.go:6289: Reusing idle postgres schema "river_2025_10_01t13_15_00_schema_03" [user facing: "river_2025_10_01t13_15_00_schema_03"] after cleaning in 744.730859ms [4 generated] [3 reused]
    logger.go:256: time=2025-10-01T13:15:01.890Z level=INFO msg="River client started" client_id=runnervm3ublj_2025_10_01T13_15_01_338625
    logger.go:256: time=2025-10-01T13:15:01.890Z level=INFO msg="inserting queue1 job"
    logger.go:256: time=2025-10-01T13:15:01.979Z level=INFO msg="received notification" topic=river_insert payload="{\"queue\": \"queue1\"}"
    logger.go:256: time=2025-10-01T13:15:01.979Z level=INFO msg="inserting queue1 job"
    logger.go:256: time=2025-10-01T13:15:02.043Z level=INFO msg="inserting queue1 job"
    logger.go:256: time=2025-10-01T13:15:02.140Z level=INFO msg="inserting queue1 job"
    logger.go:256: time=2025-10-01T13:15:02.279Z level=INFO msg="inserting queue1 job"
    logger.go:256: time=2025-10-01T13:15:02.903Z level=INFO msg="inserting queue1 job"
    logger.go:256: time=2025-10-01T13:15:02.905Z level=INFO msg="received notification" topic=river_insert payload="{\"queue\": \"queue1\"}"
    client_test.go:6350: received insert notification when it should have been debounced {topic:river_insert payload:{Queue:queue1}}
    logger.go:256: time=2025-10-01T13:15:03.394Z level=INFO msg="River client stopped" client_id=runnervm3ublj_2025_10_01T13_15_01_338625
    riverdbtest.go:289: Checked in postgres schema "river_2025_10_01t13_15_00_schema_03"; 1 idle schema(s) [5 generated] [4 reused]
FAIL
FAIL	github.com/riverqueue/river	27.292s

Start with Test_Client_InsertNotificationsAreDeduplicatedAndDebounced in client_test.go, especially the failure around lines 6289 and 6350. Run go test ./... -race -timeout 2m and use the linked CI run to compare the intermittent behavior. Done means the test no longer receives an insert notification while it should be debounced.