feat(telemetry): aggregate task completion telemetry with delta installments (#1071)

* feat(telemetry): aggregate task completion telemetry with delta installments

* fix(telemetry): prevent idle timer from breaking tests with fake timers

* fix(telemetry): guard user message decrement and fix idle re-fire logic

* fix(telemetry): cover all task launch paths for idle timer; fix pop guard

* fix(telemetry): add startIdleTelemetryCheck stub to task-run-dispatch spec

* fix(telemetry): advance baseline before emit to prevent double-reporting on throw

* fix(telemetry): address PR 1071 review feedback