telemetry: address flakiness in app-started test

[mabdinur]

Motivation

The test_app_started_is_first_message test needed to be refactored to properly handle multiple runtime_ids when processes fork. Forked processes may not submit app-started/app-closed events and may only generate metrics or logs.

Changes

Refactored test_app_started_is_first_message in tests/test_telemetry.py to:

• Use flatten_message_batches=True to simplify message handling
• Group telemetry requests by runtime_id and sort each group by seq_id to maintain correct order
• Verify that at least one runtime_id has app-started as its first message (accounting for forked processes that may not send app-lifecycle events)
• Updated docstring to explain the motivation and behavior

Workflow

1. ⚠️ Create your PR as draft ⚠️
2. Work on you PR until the CI passes
3. Mark it as ready for review
   • Test logic is modified? -> Get a review from RFC owner.
   • Framework is modified, or non obvious usage of it -> get a review from R&P team

🚀 Once your PR is reviewed and the CI green, you can merge it!

🛟 #apm-shared-testing 🛟

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

[datadog-datadog-prod-us1]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

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