[RUBY-3212] Add uniqueness validation for project name

- Updated `PafsCore::ProjectNameStep` to validate uniqueness of the project name with a custom error message.
- Added corresponding tests in `project_name_step_spec.rb` to ensure validation fails for non-unique project names.

app/steps/pafs_core/project_name_step.rb

@@ -11,6 +11,8 @@ class ProjectNameStep < BasicStep
       message: "The project name must only contain letters, underscores, hyphens and numbers"
     }
 
+    validates :name, uniqueness: { message: "The project name already exists. Your project must have a unique name." }
+
     private
 
     def step_params(params)

spec/steps/pafs_core/project_name_step_spec.rb

@@ -41,6 +41,18 @@
         end
       end
     end
+
+    context "when name is not unique" do
+      before do
+        create(:project_name_step, name: "Existing Project")
+      end
+
+      it "returns false when validation fails due to non-unique name" do
+        duplicate_params = ActionController::Parameters.new({ project_name_step: { name: "Existing Project" } })
+        expect(subject.update(duplicate_params)).to be false
+        expect(subject.errors[:name].join).to eq("The project name already exists. Your project must have a unique name.")
+      end
+    end
   end
 
   describe "#update" do