stripe: Add assertions to fix errors flagged by mypy 1.10.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

corporate/lib/stripe.py

@@ -3092,6 +3092,7 @@ class BillingSession(ABC):
                 licenses_base = ledger_entry.licenses
 
             if invoice_item_created:
+                assert invoice_period is not None
                 flat_discount, flat_discounted_months = self.get_flat_discount_info(plan.customer)
                 if plan.fixed_price is None and flat_discounted_months > 0:
                     num_months = (
@@ -3442,14 +3443,19 @@ class BillingSession(ABC):
         # the updated quantity.
         stripe_invoice = stripe.Invoice.retrieve(last_sent_invoice.stripe_invoice_id)
         assert stripe_invoice.status == "open"
+        assert isinstance(stripe_invoice.customer, str)
+        assert stripe_invoice.statement_descriptor is not None
+        assert stripe_invoice.metadata is not None
         invoice_items = stripe_invoice.lines.data
         # Stripe does something weird and puts the discount item first, so we need to reverse the order here.
         invoice_items.reverse()
         for invoice_item in invoice_items:
+            assert invoice_item.description is not None
             price_args: PriceArgs = {}
             # If amount is positive, this must be non-discount item we need to update.
             if invoice_item.amount > 0:
                 assert invoice_item.price is not None
+                assert invoice_item.price.unit_amount is not None
                 price_args = {
                     "quantity": licenses,
                     "unit_amount": invoice_item.price.unit_amount,

corporate/tests/test_stripe.py

@@ -16,6 +16,7 @@ from typing import (
     Callable,
     Dict,
     List,
+    Literal,
     Mapping,
     Optional,
     Sequence,
@@ -524,13 +525,19 @@ class StripeTestCase(ZulipTestCase):
                 },
             },
         )
+        assert isinstance(checkout_setup_intent.customer, str)
+        assert checkout_setup_intent.metadata is not None
+        assert checkout_setup_intent.usage in {"off_session", "on_session"}
+        usage = cast(
+            Literal["off_session", "on_session"], checkout_setup_intent.usage
+        )  # https://github.com/python/mypy/issues/12535
         stripe_setup_intent = stripe.SetupIntent.create(
             payment_method=payment_method.id,
             confirm=True,
             payment_method_types=checkout_setup_intent.payment_method_types,
             customer=checkout_setup_intent.customer,
             metadata=checkout_setup_intent.metadata,
-            usage=checkout_setup_intent.usage,
+            usage=usage,
         )
         [stripe_session] = iter(stripe.checkout.Session.list(customer=customer_stripe_id, limit=1))
         stripe_session_dict = orjson.loads(orjson.dumps(stripe_session))