Stabilize concurrent binding creation test

The test for concurrent service credential binding creation used threads, which caused sporadic failures in other tests due to database connection interference.
This commit rewrites the test to use a spy instead of threads.
The spy simulates the race condition by creating a conflicting binding after the database lock is acquired, verifying the code's locking mechanism without the side effects of multi-threading.

Author: johha

spec/unit/actions/service_credential_binding_app_create_spec.rb

@@ -234,20 +234,19 @@ module V3
             # TODO: Once the unique constraints to allow multiple bindings are removed, this needs to be set to 1
             # before { TestConfig.override(max_service_credential_bindings_per_app_service_instance: 1) }
 
-            def attempt_precursor
-              action.precursor(service_instance, app:, message:)
-              :ok
-            rescue StandardError => e
-              e
-            end
-
             it 'allows only one binding when two creates run in parallel' do
-              results = [Thread.new { attempt_precursor }, Thread.new { attempt_precursor }].map(&:value)
+              # This test simulates a race condition for concurrent binding creation using a spy on `service_instance`.
+              # We mock that a second binding is created after the first one acquires a lock and expect an `UnprocessableCreate` error.
+              allow(service_instance).to receive(:lock!).and_wrap_original do |m, *args, &block|
+                m.call(*args, &block)
+                ServiceBinding.make(service_instance:, app:)
+              end
+
+              expect do
+                action.precursor(service_instance, app:, message:)
+              end.to raise_error(ServiceCredentialBindingAppCreate::UnprocessableCreate, 'The app is already bound to the service instance')
 
-              expect(ServiceBinding.where(app:, service_instance:).count).to eq(1)
-              expect(results.count(:ok)).to eq(1)
-              expect(results.count { |r| r.is_a?(VCAP::CloudController::V3::ServiceBindingCreate::UnprocessableCreate) }).to eq(1)
-              expect(results.grep(Exception).map(&:message)).to include('The app is already bound to the service instance')
+              expect(service_instance).to have_received(:lock!)
             end
           end