corporate: Fix to check if the plan we are processing is paid or not.

Earlier, the 'self.on_paid_plan()' check was verifying if the
billing_session/customer is on paid plan and not the plan we
are processing.

This resulted in a bug. While processing a legacy plan, a customer
switches from legacy plan to a paid plan resulting in the
'self.on_paid_plan()' check returning True.
It leads to invoicing legacy plan which shouldn't happen.

The fix is to check if the plan we are processing is paid or not
instead of the remote_realm/remote_server plan_type.

corporate/lib/stripe.py

@@ -2762,10 +2762,10 @@ class BillingSession(ABC):
             self.make_end_of_cycle_updates_if_needed(plan, event_time)
 
         # The primary way to not create an invoice for a plan is to not have
-        # any new ledger entry. The 'self.on_paid_plan()' check adds an extra
+        # any new ledger entry. The 'plan.is_paid()' check adds an extra
         # layer of defense to avoid creating any invoices for customers not on
         # paid plan. It saves a DB query too.
-        if self.on_paid_plan():
+        if plan.is_paid():
             if plan.invoicing_status == CustomerPlan.INVOICING_STATUS_INITIAL_INVOICE_TO_BE_SENT:
                 invoiced_through_id = -1
                 licenses_base = None

corporate/models.py

@@ -390,6 +390,14 @@ class CustomerPlan(AbstractCustomerPlan):
     TIER_SELF_HOSTED_ENTERPRISE = 105
     tier = models.SmallIntegerField()
 
+    PAID_PLAN_TIERS = [
+        TIER_CLOUD_STANDARD,
+        TIER_CLOUD_PLUS,
+        TIER_SELF_HOSTED_BASIC,
+        TIER_SELF_HOSTED_BUSINESS,
+        TIER_SELF_HOSTED_ENTERPRISE,
+    ]
+
     ACTIVE = 1
     DOWNGRADE_AT_END_OF_CYCLE = 2
     FREE_TRIAL = 3
@@ -466,6 +474,9 @@ class CustomerPlan(AbstractCustomerPlan):
     def is_free_trial(self) -> bool:
         return self.status == CustomerPlan.FREE_TRIAL
 
+    def is_paid(self) -> bool:
+        return self.tier in self.PAID_PLAN_TIERS
+
 
 def get_current_plan_by_customer(customer: Customer) -> Optional[CustomerPlan]:
     return CustomerPlan.objects.filter(