feat: add testing infrastructure and helpers

[Chemaclass]

TL;DR

Adds comprehensive testing infrastructure with GacelaTestCase and ContractTestCase base classes, along with helpers and examples for testing Gacela modules and facade contracts.

Summary

• GacelaTestCase base class for testing Gacela modules
• ContractTestCase for testing facade contract compliance
• Testing helpers for common scenarios (mocking, assertions)
• Module integration testing examples
• Full documentation with best practices
• Test coverage for test infrastructure

Key Features

GacelaTestCase:

use Gacela\Testing\GacelaTestCase;

class MyModuleTest extends GacelaTestCase
{
    public function test_module_integration(): void
    {
        $facade = $this->getFacade(MyFacade::class);
        
        $result = $facade->doSomething();
        
        $this->assertInstanceOf(Result::class, $result);
    }
}

ContractTestCase:

use Gacela\Testing\ContractTestCase;

class MyFacadeContractTest extends ContractTestCase
{
    public function test_facade_implements_contract(): void
    {
        $this->assertFacadeImplementsContract(
            MyFacade::class,
            MyFacadeInterface::class
        );
    }
}

Benefits

• Consistent testing approach across modules
• Reduced boilerplate in test code
• Best practices for testing Gacela components
• Easy to mock and stub dependencies
• Contract testing for interface compliance
• Integration testing support