chore: deprecate org_id in GetCheckoutRequest

Mark org_id as deprecated in GetCheckoutRequest since checkout can be
fetched by ID alone without needing organization context.

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

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

Author: whoAbhishekSah

raystack/frontier/v1beta1/frontier.proto

@@ -2320,9 +2320,15 @@ 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
+  // DEPRECATED: org_id is not needed, checkout can be fetched by id alone
+  string org_id = 1 [
+    deprecated = true,
+    (validate.rules).string = {
+      ignore_empty: true,
+      min_len: 3
+    }
+  ];
+  // DEPRECATED: billing_id is not needed, checkout can be fetched by id alone
   string billing_id = 2 [
     deprecated = true,
     (validate.rules).string = {