Implement testing utilities foundation for Spring Boot Actor

[Copilot]

Description

Implements Spring Boot-friendly testing infrastructure by wrapping Pekko TestKit. Provides @EnableActorTesting annotation, SpringActorTestKit, and ActorTestProbe for simplified actor testing in Spring Boot contexts.

Changes Made

Core Testing Infrastructure

• @EnableActorTesting - Annotation for test auto-configuration
• SpringActorTestKit - Wraps Pekko ActorTestKit with Spring-friendly spawn/probe APIs
• ActorTestProbe - Wraps Pekko TestProbe with assertion helpers (receiveMessage(), expectMessageClass(), expectMessageThat())
• ActorTestConfiguration - Bean auto-configuration and lifecycle management

Example Usage

@SpringBootTest
@EnableActorTesting
public class OrderActorTest {
    @Autowired
    private SpringActorTestKit testKit;
    
    @Test
    public void testOrderCreation() {
        ActorTestProbe<OrderEvent> probe = testKit.createProbe();
        ActorRef<Command> actor = testKit.spawn(createBehavior(), "order");
        
        actor.tell(new CreateOrder("order-1", 100.0, probe.ref()));
        
        probe.expectMessageThat(OrderCreated.class, created -> {
            assertEquals("order-1", created.orderId());
            assertEquals(100.0, created.amount());
        });
    }
}

Documentation

• Task specifications for 8 planned features (fluent API, mock support, state verification, flow testing)
• README with usage examples and best practices
• Implementation summary with architecture overview

Additional Context

• All 140 existing tests pass unchanged
• Wraps Pekko TestKit without reimplementing core functionality
• Provides escape hatches to underlying Pekko APIs (getPekkoTestKit(), getPekkoProbe())
• Future enhancements (fluent DSL, @MockActor, state verification) documented in task specifications

Original prompt

You are tasked with implementing testing utilities for the spring-boot-starter-actor project.

REPOSITORY INFORMATION:

• Branch: main (you MUST work on main branch)
• Directory: roadmap/4-testing/

PRIORITY: HIGH EFFORT: 4 weeks

OBJECTIVE: Create Spring Boot-friendly testing utilities by wrapping Pekko TestKit. DO NOT reimplement TestKit core.

TASKS: Read all files in roadmap/4-testing/ first.

PHASE 1 (Week 1-2): Spring Boot TestKit Wrapper

• Read tasks/01-enable-actor-testing.md for detailed specifications
• Create @EnableActorTesting annotation for auto-configuration
• Create SpringActorTestKit bean that wraps Pekko TestKit
• Create auto-configuration classes for testing support
• Read tasks/02-fluent-test-api.md
• Build fluent test API wrapping Pekko TestKit (not reimplementing)
• API should support patterns like: testKit.forActor().spawn().send().expectReply()
• Read tasks/03-test-probe-support.md
• Implement ActorTestProbe wrapper
• Create message expectation helpers
• Write comprehensive tests for test utilities

PHASE 2 (Week 3): Mock Actor Support

• Read tasks/04-mock-actor-annotation.md for detailed specifications
• Create @MockActor annotation
• Integrate with Mockito for mocking actor refs
• Integrate with Spring Test framework
• Read tasks/05-mock-actor-system.md
• Create MockSpringActorSystem for programmatic mocking
• Implement behavior configuration for mocks
• Write comprehensive tests for mock support

PHASE 3 (Week 4): Common Test Utilities

• Read tasks/06-state-verification.md for detailed specifications
• Create state verification helpers (assertActorState() utilities)
• Read tasks/07-message-flow-testing.md
• Create MessageFlowTester utility
• Write comprehensive documentation for testing utilities
• Create example test cases

PHASE 4 (Deferred): Performance Testing

• Read tasks/08-performance-testing.md
• This phase is deferred (LOW priority - nice to have)
• Do NOT implement unless explicitly requested

CRITICAL IMPLEMENTATION NOTES:

• Wrap Pekko TestKit, DO NOT reimplement core testing functionality
• Focus on Spring Boot integration and developer experience
• All testing utilities must work seamlessly with @SpringBootTest
• Provide fluent APIs that feel natural to Spring Boot developers

WHAT NOT TO DO:

• DO NOT reimplement Pekko TestKit core functionality
• DO NOT create complex performance testing initially (deferred)
• DO NOT create actor visualization in tests

SUCCESS CRITERIA:

• @EnableActorTesting works seamlessly with @SpringBootTest
• Fluent test API simplifies common testing patterns
• Mock actors work correctly with Mockito
• Tests are easy to write and maintain
• No reimplementation of Pekko TestKit core

IMPLEMENTATION APPROACH: Create a thin, Spring Boot-friendly wrapper over Pekko TestKit that enhances developer experience without hiding or reimplementing core functionality.

START BY:

1. Checking out the main branch
2. Reading all files in roadmap/4-testing/
3. Reading tasks/TASK_PRIORITY.md to understand the implementation order
4. Starting with Phase 1: @EnableActorTesting annotation

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