[Flaky Test] github.com/elastic/beats/v7/x-pack/filebeat/tests/integration TestLogstashExporterProxyURL

Nobody has claimed this yet.

## Flaky Test

* **Test Name:** TestLogstashExporterProxyURL
* **Scope:** github.com/elastic/beats/v7/x-pack/filebeat/tests/integration
* **File:** N/A
* **Location:** N/A
* **Buildkite Link:** https://buildkite.com/organizations/elastic/analytics/suites/beats/tests/33a47d23-10e9-8d5c-870f-a1f0bd2616b7
* **Flaky Instances:** 1
* **Latest Occurrence:** 2026-04-08T12:06:13.027Z

### Details

```json
{
"id": "33a47d23-10e9-8d5c-870f-a1f0bd2616b7",
"web_url": "https://buildkite.com/organizations/elastic/analytics/suites/beats/tests/33a47d23-10e9-8d5c-870f-a1f0bd2616b7",
"scope": "github.com/elastic/beats/v7/x-pack/filebeat/tests/integration",
"name": "TestLogstashExporterProxyURL",
"location": null,
"file_name": null,
"instances": 1,
"latest_occurrence_at": "2026-04-08T12:06:13.027Z",
"most_recent_instance_at": "2026-04-08T12:06:13.027Z",
"last_resolved_at": null,
"ownership_team_ids": [],
"failure_examples": []
}
```

### Stacktrace

```
Failed
=== RUN TestLogstashExporterProxyURL
otel_lsexporter_test.go:252:
Error Trace: /opt/buildkite-agent/builds/bk-agent-prod-gcp-1775647715145123013/elastic/beats-xpack-filebeat/x-pack/filebeat/tests/integration/otel_lsexporter_test.go:457
/opt/buildkite-agent/builds/bk-agent-prod-gcp-1775647715145123013/elastic/beats-xpack-filebeat/x-pack/filebeat/tests/integration/otel_lsexporter_test.go:255
/opt/buildkite-agent/.asdf/installs/golang/1.25.8/go/src/runtime/asm_amd64.s:1693
Error: Not equal:
expected: 200
actual : 404
Messages: URL http://localhost:8082/78e795e4-0ab0-4249-8036-788982bb02a0_otel.json should return HTTP 200
otel_lsexporter_test.go:252:
Error Trace: /opt/buildkite-agent/builds/bk-agent-prod-gcp-1775647715145123013/elastic/beats-xpack-filebeat/x-pack/filebeat/tests/integration/otel_lsexporter_test.go:252
Error: Condition never satisfied
Test: TestLogstashExporterProxyURL
Messages: did not find Logstash output file served via Nginx
```

Start with x-pack/filebeat/tests/integration/otel_lsexporter_test.go and run TestLogstashExporterProxyURL. Inspect the assertions around lines 252, 255, and 457 and the Nginx-served output request that returns HTTP 404 instead of 200. Done means the test reliably finds the Logstash output file through the proxy URL and passes.

Written by the indexing model from the issue text.