Pipeline worker count test failed.

https://logstash-ci.elastic.co/job/elastic+logstash+master+multijob-os-compatibility/os=amazon/295/console

```
1) LogStash::Pipeline defaulting the pipeline workers based on thread safety when there are threadsafe filters only starts multiple filter threads
Failure/Error: expect(pipeline.worker_threads.size).to eq(worker_thread_count)

expected: 5
got: 8
```

Start with the linked Amazon OS compatibility CI job and the LogStash::Pipeline worker-count test named in its failure. Reproduce the mismatch between the expected five workers and the observed eight, then trace the pipeline worker-thread defaulting logic; done means the test passes consistently with the intended worker count.

Written by the indexing model from the issue text.