Add comprehensive tests for generator_helper add_npm_dependencies

Add test coverage for the add_npm_dependencies method to verify:
- Regular dependencies are added with exact: true parameter
- Dev dependencies are added with type: :dev and exact: true parameters
- Error handling returns false and prints appropriate warnings
- package_json method properly memoizes results and handles errors

These tests ensure the new exact option from package_json gem v0.2.0
is properly utilized when adding npm dependencies during generation.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: justin808

spec/react_on_rails/generators/generator_helper_spec.rb

@@ -0,0 +1,109 @@
+# frozen_string_literal: true
+
+require_relative "../support/generator_spec_helper"
+
+RSpec.describe GeneratorHelper, type: :generator do
+  include described_class
+
+  let(:destination_root) { File.expand_path("../dummy-for-generators", __dir__) }
+
+  describe "#add_npm_dependencies" do
+    context "when package_json gem is available" do
+      let(:mock_package_json) { instance_double(PackageJson) }
+      let(:mock_manager) { instance_double("PackageJson::Manager") } # rubocop:disable RSpec/VerifiedDoubleReference
+
+      before do
+        allow(self).to receive(:package_json).and_return(mock_package_json)
+        allow(mock_package_json).to receive(:manager).and_return(mock_manager)
+      end
+
+      context "when adding regular dependencies" do
+        it "calls manager.add with exact: true" do
+          packages = %w[react react-dom]
+
+          expect(mock_manager).to receive(:add).with(packages, exact: true)
+
+          result = add_npm_dependencies(packages)
+          expect(result).to be true
+        end
+      end
+
+      context "when adding dev dependencies" do
+        it "calls manager.add with type: :dev and exact: true" do
+          packages = ["@types/react", "@types/react-dom"]
+
+          expect(mock_manager).to receive(:add).with(packages, type: :dev, exact: true)
+
+          result = add_npm_dependencies(packages, dev: true)
+          expect(result).to be true
+        end
+      end
+
+      context "when package_json gem raises an error" do
+        it "returns false and prints a warning" do
+          packages = ["react"]
+
+          allow(mock_manager).to receive(:add).and_raise(StandardError, "Installation failed")
+          expect { add_npm_dependencies(packages) }.to output(/Warning: Could not add packages/).to_stdout
+
+          result = add_npm_dependencies(packages)
+          expect(result).to be false
+        end
+      end
+    end
+
+    context "when package_json gem is not available" do
+      before do
+        allow(self).to receive(:package_json).and_return(nil)
+      end
+
+      it "returns false" do
+        packages = ["react"]
+
+        result = add_npm_dependencies(packages)
+        expect(result).to be false
+      end
+    end
+  end
+
+  describe "#package_json" do
+    context "when PackageJson is available" do
+      before do
+        stub_const("PackageJson", Class.new do
+          def self.read
+            new
+          end
+        end)
+      end
+
+      it "returns a PackageJson instance" do
+        result = package_json
+        expect(result).to be_a(PackageJson)
+      end
+
+      it "memoizes the result" do
+        first_call = package_json
+        second_call = package_json
+        expect(first_call).to equal(second_call)
+      end
+    end
+
+    # NOTE: Testing the LoadError path is difficult because PackageJson is already loaded
+    # in the test environment. The StandardError path below covers the error handling logic.
+
+    context "when package.json file cannot be read" do
+      before do
+        stub_const("PackageJson", Class.new do
+          def self.read
+            raise StandardError, "File not found"
+          end
+        end)
+      end
+
+      it "returns nil and prints a warning" do
+        expect { package_json }.to output(/Warning: Could not read package\.json/).to_stdout
+        expect(package_json).to be_nil
+      end
+    end
+  end
+end