Add Minitest support with assertion-style test helpers

[pedrofs]

Summary

This PR adds comprehensive Minitest testing support alongside the existing RSpec matchers, providing the same testing capabilities for Minitest users who prefer classic assertion-style testing.

Motivation

Currently, inertia-rails only provides test helpers for RSpec. Many Rails developers use Minitest (Rails' default testing framework), and this PR brings first-class Minitest support to make testing Inertia responses easy and intuitive for those users.

Changes

New Features

• 5 assertion methods for testing Inertia responses:

  • assert_inertia_component - Assert component name matches
  • assert_inertia_exact_props - Assert props match exactly
  • assert_inertia_includes_props - Assert props include specified keys/values
  • assert_inertia_exact_view_data - Assert view data matches exactly
  • assert_inertia_includes_view_data - Assert view data includes specified keys/values

• Automatic render interception via setup/teardown hooks

• Configuration system (InertiaRails::Minitest.config)

• Warning system for missing Inertia renderers (suppressible)

• Full documentation in README.md with usage examples

Files Added/Modified

• lib/inertia_rails/minitest.rb - New Minitest test helpers implementation (160 lines)
• spec/inertia/minitest_helper_spec.rb - Comprehensive test suite (320 lines, 23 passing specs)
• README.md - Added Minitest documentation section with examples

Usage

# test/test_helper.rb
require 'inertia_rails/minitest'

class ActionDispatch::IntegrationTest
  include InertiaRails::Minitest::Helpers
end

# test/integration/events_test.rb
class EventsTest < ActionDispatch::IntegrationTest
  test "renders events index with correct data" do
    get events_path
    
    # Assert component
    assert_inertia_component 'Event/Index'
    
    # Assert props
    assert_inertia_exact_props(title: 'Events', events: [])
    assert_inertia_includes_props(title: 'Events')
    
    # Access inertia data directly
    assert_equal 'Event/Index', inertia.component
    assert_equal 'Events', inertia.props[:title]
  end
end

Implementation Details

• The implementation mirrors the existing RSpec helpers in structure and functionality
• Uses classic Minitest assertion style instead of expectation matchers
• Handles both first-load (HTML with symbol keys) and sequential (JSON with string keys) requests
• Includes the same InertiaRenderWrapper pattern used by RSpec helpers
• Zero impact on existing RSpec functionality - both can coexist

Testing

• All new Minitest helper tests pass (23 specs)
• All existing RSpec helper tests still pass (16 specs)
• No breaking changes to existing functionality

Documentation

• Added comprehensive Minitest section to README.md
• Included setup instructions, usage examples, and configuration options
• Documented the difference between symbol/string keys for different request types

Related

This addresses the need for Minitest support mentioned in the community and provides feature parity with the RSpec testing experience.