fix(auth): sanitize plan on eligiblity check

Because:

- Under certain conditions the plan eligibility check returns an
  unsanitized abbreviated plan of the current plan the customer is
  subscribed to.

This commit:

- Sanitize the plan returned by the plan eligibility API so that
  sensitive data is removed.

Closes #PAY-3275

Author: StaberindeZA

packages/fxa-auth-server/lib/routes/subscriptions/mozilla.ts

@@ -26,6 +26,7 @@ import type { AppendedPlayStoreSubscriptionPurchase } from 'fxa-shared/payments/
 import type { AppendedAppStoreSubscriptionPurchase } from 'fxa-shared/payments/iap/apple-app-store/types';
 import { VError } from 'verror';
 import type { ConfigType } from '../../../config';
+import { sanitizePlans } from './stripe';
 
 const DEFAULT_CURRENCY = 'usd';
 
@@ -227,7 +228,9 @@ export class MozillaSubscriptionHandler {
 
     return {
       eligibility: result.subscriptionEligibilityResult,
-      currentPlan: result.eligibleSourcePlan,
+      currentPlan:
+        result.eligibleSourcePlan &&
+        sanitizePlans([result.eligibleSourcePlan]).at(0),
     };
   }
 }