[system_tests] Fix flaky test_app_started_is_first_message by using seq_id ordering

[vlad-scherbich]

https://datadoghq.atlassian.net/browse/PROF-13796

Motivation

test_telemetry.py::Test_Telemetry::test_app_started_is_first_message was flaky (e.g. 8 hits in dd-trace-py main CI, 2026). The test asserted that app-started is the first telemetry message, but it used capture order (log file order) instead of send order (seq_id). Capture order can be non-deterministic—when the first captured batch is not the first sent, the test fails.

Changes

• test_app_started_is_first_message: Use seq_id as the source of truth for ordering. Flatten all lifecycle telemetry (single messages + message-batch payloads), skip non-lifecycle types (sketches, generate-metrics, logs, distributions), group by runtime_id, sort by (seq_id, batch_index), and assert per runtime that the first message is app-started. This removes dependence on capture/file order.

Reviewer checklist

• Anything but tests/ or manifests/ is modified ? I have the approval from R&P team
• A docker base image is modified?
  • the relevant build-XXX-image label is present
• A scenario is added, removed or renamed?
  • Get a review from R&P team

[github-actions]

CODEOWNERS have been resolved as:

tests/test_telemetry.py                                                 @DataDog/libdatadog-telemetry @DataDog/apm-sdk-capabilities @DataDog/system-tests-core