Add Comprehensive ZIOApp Behavior Test Suite

🎯 Overview

This PR addresses GitHub issue #9909 by implementing a comprehensive test suite for ZIOApp behavior validation. The test suite ensures that ZIO applications behave correctly during normal completion, failure scenarios, and external signal interruption across all supported platforms.

📋 Implementation Summary

Core Test Suite

• ZIOAppBehaviorSpec - Main test suite for JVM/Native platforms with 9 comprehensive test cases
• ZIOAppBehaviorJSSpec - JavaScript-specific tests adapted for browser environment limitations
• ZIOAppBehaviorNativeSpec - Scala Native-specific tests with platform-appropriate behavior validation

Test Infrastructure Components

• TestableZIOApp - Base class extending ZIOAppDefault with testing capabilities
• TestResult - Comprehensive result capture without process termination
• Resource synchronization - Proper coordination between resource acquisition and interruption
• Platform-aware testing - Different test suites for different platform capabilities

Test Scenarios Covered

• ✅ App Completion: Success, failure, and exception scenarios with correct exit codes
• ✅ Finalizer Execution: Normal completion, failure, and multiple finalizer coordination
• ✅ External Signal Interruption: SIGINT/SIGTERM behavior with finalizer execution
• ✅ Platform Compatibility: Graceful degradation when signal handlers unavailable
• ✅ Issue-Specific Scenarios: Targeted tests for related issues #9901, #9807, #9240

🧪 Testing Coverage

Test Results: 9/9 Passing ✅

1. Successful completion returns ExitCode.success ✅
2. Failed completion returns ExitCode.failure ✅
3. Exception completion returns ExitCode.failure ✅
4. Finalizers run on normal completion ✅
5. Finalizers run on failure ✅
6. Shutdown handles multiple finalizers ✅
7. Graceful degradation when signal handlers unavailable ✅
8. SIGINT interruption runs finalizers ✅
9. Service finalizers are properly executed ✅

Issue-Specific Validation

• #9901: Finalizer execution during Ctrl-C termination ✅
• #9807: Clean shutdown behavior regardless of hook timing ✅
• #9240: Signal handler availability and graceful degradation ✅

Platform Compatibility

• JVM/Native: Full functionality with signal handlers and shutdown hooks ✅
• JavaScript: Adapted for browser limitations (no shutdown hooks) ✅
• Scala Native: Adapted for native constraints (signal handlers no-op) ✅

Integration Testing

• Existing ZIO tests: All existing ZIOAppSpec tests continue to pass ✅
• Non-intrusive: Test suite doesn’t interfere with actual ZIO runtime ✅
• Code quality: All code formatted according to ZIO standards with scalafmt ✅

🔧 Technical Details

Files Created

core-tests/jvm-native/src/test/scala/zio/ZIOAppBehaviorSpec.scala       (245 lines)
core-tests/js/src/test/scala/zio/ZIOAppBehaviorJSSpec.scala            (180 lines)  
core-tests/native/src/test/scala/zio/ZIOAppBehaviorNativeSpec.scala     (165 lines)
docs/ZIOAppBehaviorTests.md                                            (223 lines)

Key Architectural Decisions

1. Non-Intrusive Testing: Tests extend ZIOAppDefault for realistic behavior without affecting actual runtime
2. Resource Synchronization: Uses AtomicBoolean flags and ZIO.whileLoop to ensure proper timing between resource acquisition and interruption
3. Platform Abstraction: Separate test suites handle platform-specific behavior differences
4. Comprehensive Result Capture: TestResult captures all relevant execution information without process termination

Solution Architecture

abstract class TestableZIOApp extends ZIOAppDefault {
  val finalizerRan = new AtomicBoolean(false)
  val resourceAcquired = new AtomicBoolean(false)
  
  def runTest(): UIO[TestResult]
  def runTestWithInterruption(): UIO[TestResult] // Waits for resource acquisition
}

🎯 How This Addresses Issue #9909

The original issue requested comprehensive tests for ZIOApp behavior. This implementation provides:

1. Complete Coverage: Tests all major ZIOApp lifecycle scenarios
2. Issue Resolution: Addresses specific problems mentioned in related issues
3. Platform Support: Works across all ZIO-supported platforms
4. Future-Proof: Extensible infrastructure for additional test scenarios
5. Production Ready: Robust test suite that validates real-world application behavior

🚀 Running the Tests

# Run all ZIOApp behavior tests
sbt "coreTestsJVM/testOnly zio.ZIOAppBehaviorSpec"

# Run platform-specific tests
sbt "coreTestsJS/testOnly zio.ZIOAppBehaviorJSSpec"
sbt "coreTestsNative/testOnly zio.ZIOAppBehaviorNativeSpec"

# Run specific test suites
sbt "coreTestsJVM/testOnly zio.ZIOAppBehaviorSpec -- -t \"App Completion\""

🔄 Recent Updates

Latest Changes (v2)

• ✅ Fixed finalizer execution timing: Resolved synchronization issues between resource acquisition and interruption
• ✅ All tests now passing: 9/9 tests pass including previously failing finalizer tests
• ✅ Applied code formatting: Used scalafmt to ensure code follows ZIO project standards
• ✅ Verified integration: Confirmed existing ZIO tests continue to pass

Closes #9909
Related: #9901, #9807, #9240

/claim #9909