migrate libbeat monitoring tests from python to go

[leehinman]

Proposed commit message

Converts test_monitoring.py system tests to Go integration tests using the existing libbeat/tests/integration framework, continuing the effort to replace Python-based system tests with Go integration tests.

• Adds libbeat/tests/integration/monitoring_test.go with three tests
• Removes libbeat/tests/system/test_monitoring.py

The three tests cover:

Go test	Python original	Infrastructure required
TestMonitoringClusterUUID	test_cluster_uuid_setting	None
TestMonitoringClusterUUIDMonitoringDisabled	test_cluster_uuid_setting_monitoring_disabled	None
TestMonitoringDirectToCluster	test_direct_to_monitoring_cluster	Main ES + monitoring ES

Notable implementation details

Dynamic HTTP port allocation — The Python tests hardcoded localhost:5066 for the beat's HTTP /state endpoint. The Go tests use freePort() to allocate a random available port and configure it in the beat's YAML config. This prevents port conflicts when a test process is left alive after a timeout.

Selective skipping for the monitoring cluster — TestMonitoringDirectToCluster calls EnsureESIsRunning (fatal if the main cluster is absent, consistent with all other ES integration tests) and ensureMonitoringESIsRunning (skip if the dedicated monitoring cluster at ES_MONITORING_HOST:ES_MONITORING_PORT is absent). This matches the original Python behaviour where that test was only run when INTEGRATION_TESTS was set and the monitoring cluster was available.

Original class was @unittest.skip — The entire Python class was marked permanently skipped due to flakiness (#16247). The two cluster UUID tests translate cleanly into reliable Go tests that need no external services. TestMonitoringDirectToCluster is a proper integration test gated on the full monitoring infrastructure.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works. Where relevant, I have used the stresstest.sh script to run them under stress conditions and race detector to verify their stability.
• I have added an entry in ./changelog/fragments using the changelog tool.

Disruptive User Impact

None. Test code.

How to test this PR locally

cd libbeat && mage goIntegTest

Related issues

• Relates [9.0](backport #44813) fix: AWS S3 improve unexpected EOF detection #44850

Use cases

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