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

Author: edelauna

src/core/task/Task.ts

@@ -4782,12 +4782,11 @@ export class Task extends EventEmitter<TaskEvents> implements TaskLike {
 
 	startIdleTelemetryCheck(): void {
 		this.idleTelemetryCheckInterval = setInterval(() => {
-			// lastMessageTs only moves forward on activity, so comparing it against the
-			// last flush tells us whether anything happened since that flush -- if the
-			// task has been quiet since well before the last flush, there's nothing new
-			// to report and flushTelemetryInstallment's own empty-check would no-op anyway,
-			// but skipping here avoids waking up to do that check needlessly.
-			const idleForMs = Date.now() - (this.lastMessageTs ?? this.lastTelemetryFlushAt)
+			// Measure idleness from the later of the last activity and the last flush.
+			// Using lastMessageTs alone would keep the condition true forever after the
+			// first idle flush, re-running the empty-delta check on every interval tick.
+			const lastEventAt = Math.max(this.lastMessageTs ?? 0, this.lastTelemetryFlushAt)
+			const idleForMs = Date.now() - lastEventAt
 
 			if (idleForMs >= Task.IDLE_TELEMETRY_THRESHOLD_MS) {
 				this.flushTelemetryInstallment("idle")

src/core/task/__tests__/Task.spec.ts

@@ -3118,7 +3118,13 @@ describe("Telemetry installments (idle/shutdown flush)", () => {
 		}
 	})
 
+	const createdTasks: Task[] = []
+
 	afterEach(() => {
+		for (const task of createdTasks) {
+			task.dispose()
+		}
+		createdTasks.length = 0
 		vi.useRealTimers()
 		captureTaskCompletedSpy.mockRestore()
 	})
@@ -3131,6 +3137,7 @@ describe("Telemetry installments (idle/shutdown flush)", () => {
 			startTask: false,
 		})
 		task.startIdleTelemetryCheck()
+		createdTasks.push(task)
 		return task
 	}