Drain pending telemetry before CLI shutdown

[Copilot]

TelemetryHostMatrixTests.BuildTelemetryIsExportedForEachHost intermittently timed out because command/finish could remain queued while shutdown stopped and exported the main activity.

• Fix shutdown ordering
  • Wait for queued telemetry events before stopping the activity and flushing providers.
  • Prevent command/finish from being dropped during process teardown.

TelemetryClient.Instance?.WaitForPendingEvents();
s_mainActivity?.Stop();
TelemetryClient.FlushProviders();

• Fixes [AI discovered CI] TelemetryHostMatrixTests.BuildTelemetryIsExportedForEachHost times out waiting for build telemetry #55982

[azure-pipelines]

Azure Pipelines:
3 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.