test: Flaky TestRunManager_Reloader_newRunnerError

```
[73](https://github.com/elastic/apm-server/actions/runs/30850619181/job/91809463500#step:5:174)
=== RUN TestRunManager_Reloader_newRunnerError
logger.go:146: 2026-08-03T20:33:02.380Z DEBUG manager.centralmgmt.V2-manager Manager starting
logger.go:146: 2026-08-03T20:33:02.383Z DEBUG manager.centralmgmt.V2-manager Listening for agent unit changes
logger.go:146: 2026-08-03T20:33:02.383Z INFO manager.centralmgmt.V2-manager BeatV2Manager.unitListen UnitChanged.ID(output-unit), UnitChanged.Type(added), UnitChanged.Trigger(0): added/nothing_triggered
logger.go:146: 2026-08-03T20:33:02.383Z DEBUG manager.centralmgmt.V2-manager Debounce timer fired, but Beat is not ready yet.
logger.go:146: 2026-08-03T20:33:02.383Z INFO manager.centralmgmt.V2-manager BeatV2Manager.unitListen UnitChanged.ID(input-unit-1), UnitChanged.Type(added), UnitChanged.Trigger(0): added/nothing_triggered
logger.go:146: 2026-08-03T20:33:02.383Z DEBUG manager.centralmgmt.V2-manager Debounce timer fired, but Beat is not ready yet.
logger.go:146: 2026-08-03T20:33:02.384Z DEBUG manager.centralmgmt.V2-manager Manager ready to accept units.
logger.go:146: 2026-08-03T20:33:12.384Z DEBUG manager.centralmgmt.V2-manager Got output unit config 'default'
logger.go:146: 2026-08-03T20:33:12.384Z INFO beat loaded output config
logger.go:146: 2026-08-03T20:33:12.384Z DEBUG manager.centralmgmt.V2-manager Skipped reloading APM tracing; configuration didn't change
beat_test.go:639: timed out waiting for input failed msg
--- FAIL: TestRunManager_Reloader_newRunnerError (10.00s)
```

https://github.com/elastic/apm-server/actions/runs/30850619181/job/91809463500

Start with TestRunManager_Reloader_newRunnerError in beat_test.go, especially the timeout reported near line 639. Run the test repeatedly and review the manager and reloader logs shown in the failure. Done means the test reliably observes the expected input failure without timing out.

Written by the indexing model from the issue text.