add createUser mutation

spec/graphql/mutations/create_user_spec.rb

@@ -0,0 +1,66 @@
+# frozen_string_literal: true
+require "rails_helper"
+
+RSpec.describe(Mutations::CreateUser) do
+  describe "#resolve" do
+    let(:mutation_string) do
+      <<~GQL
+        mutation($input: CreateUserInput!) {
+          createUser(input: $input) {
+            success
+            errors
+          }
+        }
+      GQL
+    end
+
+    let(:context) do
+      {
+        current_auth: Auth::Profile.new({
+          id: "_",
+          email: "user@example.com",
+        }),
+      }
+    end
+
+    let(:variables) do
+      {
+        input: { user: {
+          firstName: "First Name",
+          lastName: "Last Name",
+        } },
+      }
+    end
+
+    context "when current_auth is not being used by any user" do
+      it "create a user to auth" do
+        result = XStakeSchema.execute(
+          mutation_string,
+          variables: variables,
+          context: context
+        ).to_h
+
+        expect(result["data"]["createUser"]["success"]).to(eq(true))
+      end
+    end
+
+    context "when auth is being used by no users" do
+      it "returns error" do
+        User.create(
+          first_name: "First Name",
+          last_name: "Last Name",
+          email: "user@example.com"
+        )
+
+        result = XStakeSchema.execute(
+          mutation_string,
+          variables: variables,
+          context: context
+        ).to_h
+
+        expect(result["data"]["createUser"]["success"]).to(eq(false))
+        expect(result["data"]["createUser"]["errors"]).to(eq(["Validation failed: Email has already been taken"]))
+      end
+    end
+  end
+end