TestInput/Default_stream

## Flaky Test

* **Test Name:** TestInput/Default_stream
* **Link:** https://github.com/elastic/beats/blob/58f44bb27b806ba7f23cf1956a2590f4a3fe744a/x-pack/filebeat/input/unifiedlogs/input_test.go#L259
* **Branch:** `main`. [PR](https://github.com/elastic/beats/pull/45124/)
* **Artifact Link:** https://github.com/elastic/beats/actions/runs/16298525193/job/46026682953?pr=45124#step:3:625
* **Notes:** It is failing on MacOS 15, but it passes when I run it locally.

### Stack Trace

```
=== Failed
=== FAIL: x-pack/filebeat/input/unifiedlogs TestInput/Default_stream (65.02s)
input_test.go:254:
Error Trace: /Users/runner/work/beats/beats/x-pack/filebeat/input/unifiedlogs/input_test.go:77
/Users/runner/work/beats/beats/x-pack/filebeat/input/unifiedlogs/input_test.go:259
/Users/runner/hostedtoolcache/go/1.24.4/arm64/src/runtime/asm_arm64.s:1223
Error: Should NOT be empty, but was []
input_test.go:254:
Error Trace: /Users/runner/work/beats/beats/x-pack/filebeat/input/unifiedlogs/input_test.go:254
Error: Condition never satisfied
Test: TestInput/Default_stream

=== FAIL: x-pack/filebeat/input/unifiedlogs TestInput (113.09s)
```

Start with x-pack/filebeat/input/unifiedlogs/input_test.go, especially the checks around lines 77, 254, and 259, and review PR 45124 for the failing TestInput/Default_stream case. Reproduce the failure on macOS 15 and compare it with a local run; done means the test passes reliably in CI without the empty output or timeout failures.

Written by the indexing model from the issue text.