openapi: fix invalid BillingStatus schema (object + enum hybrid)

The previous PR (#14070) inadvertently left BillingStatus as an
invalid hybrid: an OpenAPI object schema with both 'properties' and
'enum' — the enum is meaningless on an object type.

The object properties (subscription, balance, has_payment_method)
duplicate what's already in BillingStatusResponse, the only place
that referenced BillingStatus did so via 'billing_status' field
which clearly wants the enum value.

Cleanly converts BillingStatus to a string enum matching cloud's
runtime emission. Closes the last (1) handler reference in
Comfy-Org/cloud's TestCutoverSafe gate (BE-1106).

openapi.yaml

@@ -9585,16 +9585,9 @@ components:
           description: List of plan features
 
     BillingStatus:
-      type: object
+      type: string
       x-runtime: [cloud]
-      description: "[cloud-only] Overall billing and subscription status."
-      properties:
-        subscription:
-          $ref: "#/components/schemas/BillingSubscription"
-        balance:
-          $ref: "#/components/schemas/BillingBalance"
-        has_payment_method:
-          type: boolean
+      description: "[cloud-only] Overall billing/payment lifecycle status."
       enum:
         - awaiting_payment_method
         - pending_payment