add telemetry event for subscription cancellation (#6684)

emits event after going to dashboard and returning to page and having
subscription status change from subscribed to not subscribed.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-6684-add-telemetry-event-for-subscription-cancellation-2aa6d73d365081009770de6d1db2b701)
by [Unito](https://www.unito.io)

src/platform/telemetry/providers/cloud/MixpanelTelemetryProvider.ts

@@ -175,6 +175,14 @@ export class MixpanelTelemetryProvider implements TelemetryProvider {
     this.trackEvent(TelemetryEvents.MONTHLY_SUBSCRIPTION_SUCCEEDED)
   }
 
+  /**
+   * Track when a user completes a subscription cancellation flow.
+   * Fired after we detect the backend reports `is_active: false` and the UI stops polling.
+   */
+  trackMonthlySubscriptionCancelled(): void {
+    this.trackEvent(TelemetryEvents.MONTHLY_SUBSCRIPTION_CANCELLED)
+  }
+
   trackApiCreditTopupButtonPurchaseClicked(amount: number): void {
     const metadata: CreditTopupMetadata = {
       credit_amount: amount