Fix intermittent test Test_Client_Maintenance/JobRescuer

[brandur]

Fix an intermittent test for the client maintenance services observed
from time to time [1].

--- FAIL: Test_Client_Maintenance (0.00s)
    --- FAIL: Test_Client_Maintenance/JobRescuer (4.56s)
        client_test.go:3866: Generated schema "river_test_2025_04_26t22_05_48_schema_21" with migrations [1 2 3 4 5 6] on line "main" in 1.349501729s [21 generated] [14 reused]
        logger.go:256: time=2025-04-26T22:05:52.463Z level=INFO msg="River client started" client_id=fv-az2241-783_2025_04_26T22_05_52_429607
        logger.go:256: time=2025-04-26T22:05:52.638Z level=ERROR msg="maintenance.JobRescuer: Attempted to rescue unhandled job kind, discarding" job_kind=fake_job job_id=6
        logger.go:256: time=2025-04-26T22:05:53.228Z level=INFO msg="maintenance.JobRescuer: Ran successfully" num_jobs_discarded=1 num_jobs_retry_scheduled=2
        logger.go:256: time=2025-04-26T22:05:53.253Z level=INFO msg="maintenance.JobScheduler: Ran successfully" num_jobs_scheduled=2
        client_test.go:3912:
                Error Trace:	/home/runner/work/river/river/client_test.go:3903
                                                        /home/runner/work/river/river/client_test.go:3912
                Error:      	Not equal:
                                expected: "retryable"
                                actual  : "completed"

                                Diff:
                                --- Expected
                                +++ Actual
                                @@ -1,2 +1,2 @@
                                -(rivertype.JobState) (len=9) "retryable"
                                +(rivertype.JobState) (len=9) "completed"

                Test:       	Test_Client_Maintenance/JobRescuer
        logger.go:256: time=2025-04-26T22:05:55.631Z level=INFO msg="River client stopped" client_id=fv-az2241-783_2025_04_26T22_05_52_429607
        riverdbtest.go:277: Checked in schema "river_test_2025_04_26t22_05_48_schema_21"; 1 idle schema(s) [22 generated] [15 reused]
FAIL
FAIL	github.com/riverqueue/river	32.635s

This one's similar to #852 in that it's possible for test case goroutine
to be paused long enough (really only happens in GitHub Actions CI) that
a job it moved to retryable actually becomes eligible to be worked,
and may actually be worked in the interim before the test case picks up
again to assert on it.

The solution is to use the new RetryPolicySlow introduced in #852 to
make sure that retryable jobs are scheduled far out in the future and
never at risk of really being retried in the test case.

We also make the slow retry interval's default a little longer. The test
sets job's attempted_at (which its next scheduled_at will be based
on) an hour in the past which is exactly what the slow retry interval
was, making jobs still potentially immediately retryable. Make the slow
interval two hours so contention is never possible.

The good news is that this is the last intermittent test I've been
tracking. I'm hoping that it along with the fixes from Saturday might
be enough to take care of our intermittency problems for a while.

[1] https://github.com/riverqueue/river/actions/runs/14685519857/job/41213612537

[brandur]

Sigh. Didn't fix it.

[brandur]

@bgentry Okay, problem turned out to be because the test was setting attempted_at an hour in the past, so adding the slow retry policy's 1 hour interval was bringing the next retry back the current time, so the job would still have a chance of running immediately and failing the test. I changed the slow retry policy's interval to two hours and that should fix it. Seem okay?

[bgentry]

hah, nice work again 🕵️‍♂️

[brandur]

ty ty. Hopefully the last one!