[RUBY-3212] Refactor project name uniqueness validation in spec

- Moved the creation of `existing_project` to a `before` block for better readability and consistency.

spec/steps/pafs_core/project_name_step_spec.rb

@@ -46,9 +46,10 @@
     end
 
     context "when name already exists" do
-      let!(:existing_project) { create(:project_name_step, name: "Unique Project Name") }
       let(:duplicate_name_params) { ActionController::Parameters.new({ project_name_step: { name: "Unique Project Name" } }) }
 
+      before { create(:project_name_step, name: "Unique Project Name") }
+
       it "returns false and sets the uniqueness error message" do
         project_name_step.name = "Different Name"
         expect(project_name_step.update(duplicate_name_params)).to be false