Closes https://github.com/coder/internal/issues/677
Resolves flakes such as:
```
$ go test -race -run "TestRunStopRace" github.com/coder/coder/v2/coderd/notifications -count=10000 -parallel $(nproc)
--- FAIL: TestRunStopRace (0.00s)
t.go:106: 2025-06-06 02:44:39.348 [debu] notifications-manager: notification manager started
t.go:106: 2025-06-06 02:44:39.348 [debu] notifications-manager: graceful stop requested
t.go:106: 2025-06-06 02:44:39.348 [debu] notifications-manager: notification manager stopped
t.go:115: 2025-06-06 02:44:39.348 [erro] notifications-manager: notification manager stopped with error ...
error= manager already closed:
github.com/coder/coder/v2/coderd/notifications.(*Manager).loop
/home/coder/coder/coderd/notifications/manager.go:166
*** slogtest: log detected at level ERROR; TEST FAILURE ***
--- FAIL: TestRunStopRace (0.00s)
t.go:106: 2025-06-06 02:44:41.632 [debu] notifications-manager: notification manager started
t.go:106: 2025-06-06 02:44:41.632 [debu] notifications-manager: graceful stop requested
t.go:106: 2025-06-06 02:44:41.632 [debu] notifications-manager: notification manager stopped
t.go:115: 2025-06-06 02:44:41.633 [erro] notifications-manager: notification manager stopped with error ...
error= manager already closed:
github.com/coder/coder/v2/coderd/notifications.(*Manager).loop
/home/coder/coder/coderd/notifications/manager.go:166
*** slogtest: log detected at level ERROR; TEST FAILURE ***
FAIL
FAIL github.com/coder/coder/v2/coderd/notifications 6.847s
FAIL
```
These error logs are caused as a result of the `Manager` `Run` start operation being asynchronous. In the flaking test case we immediately call `Stop` after `Run`. It's possible for `Stop` to be scheduled to completion before the goroutine spawned by `Run` calls `loop` and checks `closed`. If this happens, `loop` returns an error and produces the error log.
We'll address this by replacing this specific error log with a warning log.
```
$ go test -run "TestRunStopRace" github.com/coder/coder/v2/coderd/notifications -count=10000 -parallel $(nproc)
ok github.com/coder/coder/v2/coderd/notifications 1.294s
$ go test -race github.com/coder/coder/v2/coderd/notifications -count=100 -parallel $(nproc)
ok github.com/coder/coder/v2/coderd/notifications 26.525s
```
533c6dcbbe