corporate: Improve testing for switching from Standard to Plus.

This is a follow-up to #19752. The tests in that PR did not verify
that the financial math involved worked properly. This commit
improves the existing tests and adds new fixtures to make sure
that the financial math works as expected.

corporate/tests/test_stripe.py

@@ -300,6 +300,7 @@ MOCKED_STRIPE_FUNCTION_NAMES = [
         "Coupon.create",
         "Customer.create",
         "Customer.create_balance_transaction",
+        "Customer.list_balance_transactions",
         "Customer.retrieve",
         "Customer.save",
         "Invoice.create",
@@ -2982,7 +2983,8 @@ class StripeTest(StripeTestCase):
 
     @mock_stripe()
     def test_switch_realm_from_standard_to_plus_plan(self, *mock: Mock) -> None:
-        realm = get_realm("zulip")
+        iago = self.example_user("iago")
+        realm = iago.realm
 
         # Test upgrading to Plus when realm has no Standard subscription
         with self.assertRaises(BillingError) as billing_context:
@@ -3002,12 +3004,28 @@ class StripeTest(StripeTestCase):
             "Organization missing Stripe customer.", billing_context.exception.error_description
         )
 
-        plan.customer.stripe_customer_id = "cus_12345"
-        plan.customer.save(update_fields=["stripe_customer_id"])
-        plan.price_per_license = get_price_per_license(CustomerPlan.STANDARD, CustomerPlan.MONTHLY)
-        plan.automanage_licenses = True
+        king = self.lear_user("king")
+        realm = king.realm
+        customer = update_or_create_stripe_customer(king)
+        plan = CustomerPlan.objects.create(
+            customer=customer,
+            automanage_licenses=True,
+            billing_cycle_anchor=timezone_now(),
+            billing_schedule=CustomerPlan.MONTHLY,
+            tier=CustomerPlan.STANDARD,
+        )
+        ledger = LicenseLedger.objects.create(
+            plan=plan,
+            is_renewal=True,
+            event_time=timezone_now(),
+            licenses=9,
+            licenses_at_next_renewal=9,
+        )
+        realm.plan_type = Realm.PLAN_TYPE_STANDARD
+        realm.save(update_fields=["plan_type"])
         plan.invoiced_through = ledger
-        plan.save(update_fields=["price_per_license", "automanage_licenses", "invoiced_through"])
+        plan.price_per_license = get_price_per_license(CustomerPlan.STANDARD, CustomerPlan.MONTHLY)
+        plan.save(update_fields=["invoiced_through", "price_per_license"])
 
         switch_realm_from_standard_to_plus_plan(realm)
 
@@ -3018,6 +3036,29 @@ class StripeTest(StripeTestCase):
         self.assertEqual(plus_plan.tier, CustomerPlan.PLUS)
         self.assertEqual(LicenseLedger.objects.filter(plan=plus_plan).count(), 1)
 
+        # There are 9 licenses and the realm is on the Standard monthly plan.
+        # Therefore, the customer has already paid 800 * 9 = 7200 = $72 for
+        # the month. Once they upgrade to Plus, the new price for their 9
+        # licenses will be 1600 * 9 = 14400 = $144. Since the customer has
+        # already paid $72 for a month, -7200 = -$72 will be credited to the
+        # customer's balance.
+        stripe_customer_id = customer.stripe_customer_id
+        assert stripe_customer_id is not None
+        _, cb_txn = stripe.Customer.list_balance_transactions(  # type: ignore[attr-defined] # mypy seems to incorrectly think that this function doesn't exist
+            stripe_customer_id
+        )
+        self.assertEqual(cb_txn.amount, -7200)
+        self.assertEqual(
+            cb_txn.description,
+            "Credit from early termination of Standard plan",
+        )
+        self.assertEqual(cb_txn.type, "adjustment")
+
+        # The customer now only pays the difference 14400 - 7200 = 7200 = $72,
+        # since the unused proration is for the whole month.
+        (invoice,) = stripe.Invoice.list(customer=stripe_customer_id)
+        self.assertEqual(invoice.amount_due, 7200)
+
     def test_update_billing_method_of_current_plan(self) -> None:
         realm = get_realm("zulip")
         customer = Customer.objects.create(realm=realm, stripe_customer_id="cus_12345")