Use exact option in package_json gem for installing dependencies (#1939)

## Summary

Updates the `add_npm_dependencies` helper method to use the new `exact:
true` option from package_json gem v0.2.0, ensuring npm packages are
installed with exact versions (no ^ or ~ prefix).

## Changes

- Modified `lib/generators/react_on_rails/generator_helper.rb` to pass
`exact: true` when calling `pj.manager.add()` for both regular and dev
dependencies

## Motivation

The package_json gem v0.2.0 (released 2025-11-06) now supports an
`exact` parameter in the `add` method to install packages with exact
versions. This aligns with React on Rails' requirement for strict
version matching between the gem and npm packages, eliminating the need
for manual version specification cleanup.

## Test plan

- RuboCop passes with no offenses
- Generator tests verify package installation behavior
- Existing functionality preserved with enhanced exact version
specification

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

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<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Chores**
* Enhanced npm dependency resolution during project generation to
enforce exact version matching for consistency and reliability.

* **Tests**
* Added comprehensive test coverage for dependency management and error
handling workflows.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Claude <[email protected]>

Author: justin808

lib/generators/react_on_rails/generator_helper.rb

@@ -26,9 +26,9 @@ def add_npm_dependencies(packages, dev: false)
 
     begin
       if dev
-        pj.manager.add(packages, type: :dev)
+        pj.manager.add(packages, type: :dev, exact: true)
       else
-        pj.manager.add(packages)
+        pj.manager.add(packages, exact: true)
       end
       true
     rescue StandardError => e

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