chore: deprecate billing_id in checkout RPCs

Mark billing_id as deprecated in all checkout request messages:
- CreateCheckoutRequest
- ListCheckoutsRequest
- GetCheckoutRequest
- DelegatedCheckoutRequest (admin)

Backend will infer billing_id from org_id automatically.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: whoAbhishekSah

raystack/frontier/v1beta1/admin.proto

@@ -1033,11 +1033,15 @@ message RemovePlatformUserResponse {}
 
 message DelegatedCheckoutRequest {
   string org_id = 1 [(validate.rules).string.min_len = 3];
+  // DEPRECATED: billing_id will be inferred from org_id
   // ID of the billing account to update the subscription for
-  string billing_id = 2 [(validate.rules).string = {
-    ignore_empty: true,
-    uuid: true
-  }];
+  string billing_id = 2 [
+    deprecated = true,
+    (validate.rules).string = {
+      ignore_empty: true,
+      uuid: true
+    }
+  ];
 
   // Subscription to create
   CheckoutSubscriptionBody subscription_body = 10;

raystack/frontier/v1beta1/frontier.proto

@@ -2275,11 +2275,15 @@ message CheckCreditEntitlementResponse {
 
 message CreateCheckoutRequest {
   string org_id = 1 [(validate.rules).string.min_len = 3];
+  // DEPRECATED: billing_id will be inferred from org_id
   // ID of the billing account to update the subscription for
-  string billing_id = 2  [(validate.rules).string = {
-    ignore_empty: true,
-    uuid: true
-  }];
+  string billing_id = 2  [
+    deprecated = true,
+    (validate.rules).string = {
+      ignore_empty: true,
+      uuid: true
+    }
+  ];
 
   string success_url = 3;
   string cancel_url = 4;
@@ -2299,11 +2303,15 @@ message CreateCheckoutResponse {
 
 message ListCheckoutsRequest {
   string org_id = 1 [(validate.rules).string.min_len = 3];
+  // DEPRECATED: billing_id will be inferred from org_id
   // ID of the billing account to get the subscriptions for
-  string billing_id = 2 [(validate.rules).string = {
-    ignore_empty: true,
-    uuid: true
-  }];
+  string billing_id = 2 [
+    deprecated = true,
+    (validate.rules).string = {
+      ignore_empty: true,
+      uuid: true
+    }
+  ];
 }
 
 message ListCheckoutsResponse {
@@ -2313,11 +2321,15 @@ message ListCheckoutsResponse {
 
 message GetCheckoutRequest {
   string org_id = 1 [(validate.rules).string.min_len = 3];
+  // DEPRECATED: billing_id will be inferred from org_id
   // ID of the billing account to get the subscriptions for
-  string billing_id = 2 [(validate.rules).string = {
-    ignore_empty: true,
-    uuid: true
-  }];
+  string billing_id = 2 [
+    deprecated = true,
+    (validate.rules).string = {
+      ignore_empty: true,
+      uuid: true
+    }
+  ];
   // ID of the checkout to get
   string id = 3 [(validate.rules).string.min_len = 1];
 }