[AC-2959] ACH Direct Debit POC (#4703)

* Refactor: Rename some methods and models for consistency

This commit contains no logic changes at all. It's entirely comprised of renames of existing models and methods to bring our codebase more in line with our app's functionality and terminology.

* Add feature flag: AC-2476-deprecate-stripe-sources-api

* Standardize error responses from applicable billing controllers

During my work on CB, I found that just using the built-in TypedResults errors results in the client choking on the response because it's looking for the ErrroResponseModel. The new BaseBillingController provides Error utilities to return TypedResults wrapping that model so the client can process it.

* Add feature flagged payment method endoints to OrganizationBillingController

* Run dotnet format

src/Api/Billing/Controllers/ProviderClientsController.cs

@@ -39,7 +39,7 @@ public class ProviderClientsController(
 
         if (user == null)
         {
-            return UnauthorizedResponse();
+            return Error.Unauthorized();
         }
 
         var organizationSignup = new OrganizationSignup
@@ -96,7 +96,7 @@ public class ProviderClientsController(
 
         if (providerOrganization == null)
         {
-            return NotFoundResponse();
+            return Error.NotFound();
         }
 
         var clientOrganization = await organizationRepository.GetByIdAsync(providerOrganization.OrganizationId);