[Flaky Test] github.com/elastic/beats/v7/x-pack/heartbeat/monitors/browser/synthexec TestRunBadExitCodeCmd

## Flaky Test

* **Test Name:** TestRunBadExitCodeCmd
* **Scope:** github.com/elastic/beats/v7/x-pack/heartbeat/monitors/browser/synthexec
* **File:** N/A
* **Location:** N/A
* **Buildkite Link:** https://buildkite.com/organizations/elastic/analytics/suites/beats/tests/9acde42a-28b8-8dc2-ad72-7426bd337860
* **Flaky Instances:** 1
* **Latest Occurrence:** 2026-08-25T04:17:08.588Z

### Details

```json
{
"id": "9acde42a-28b8-8dc2-ad72-7426bd337860",
"web_url": "https://buildkite.com/organizations/elastic/analytics/suites/beats/tests/9acde42a-28b8-8dc2-ad72-7426bd337860",
"scope": "github.com/elastic/beats/v7/x-pack/heartbeat/monitors/browser/synthexec",
"name": "TestRunBadExitCodeCmd",
"location": null,
"file_name": null,
"instances": 1,
"latest_occurrence_at": "2026-08-25T04:17:08.588Z",
"most_recent_instance_at": "2026-08-25T04:17:08.588Z",
"last_resolved_at": null,
"ownership_team_ids": [],
"failure_examples_count": 1
}
```

### Failure Examples

**Example 1:**
**Run:** https://api.buildkite.com/v2/analytics/organizations/elastic/suites/beats/runs/8e3026ff-8ef3-8326-a37a-24a88af5c642
**Time:** 2026-08-25T04:10:14.333Z
**Stacktrace:**

```
=== RUN TestRunBadExitCodeCmd
synthexec_test.go:245:
Error Trace: /opt/buildkite-agent/builds/bk-agent-prod-gcp-1787630743885569809/elastic/beats-xpack-heartbeat/x-pack/heartbeat/monitors/browser/synthexec/synthexec_test.go:245
Error: "[0x30be4b6d0210]" should have 2 item(s), but has 1
Test: TestRunBadExitCodeCmd
--- FAIL: TestRunBadExitCodeCmd (0.73s)

```

Start with x-pack/heartbeat/monitors/browser/synthexec/synthexec_test.go at line 245 and run TestRunBadExitCodeCmd. Compare the local failure with the linked Buildkite run to identify why the assertion sometimes sees one item instead of two. Done means the test passes reliably without hiding the intended bad-exit-code behavior.

Written by the indexing model from the issue text.