feat: Implement comprehensive Testing Architecture Enhancement

Establishes robust testing framework with clean architecture patterns:

## Phase 1: Test Infrastructure Foundation
- Global test configuration with pytest.ini and conftest.py
- Isolated test workspaces and environment management
- Comprehensive fixture library for all test types
- Test requirements and dependency management

## Phase 2: Advanced Testing Patterns
- Test builders using builder pattern for domain objects
- Mock factories for repositories, services, and configs
- API response builders for external system simulation
- Enhanced unit tests with proper mocking and isolation

## Phase 3: Test Performance and Quality
- Performance testing framework with benchmarks
- Memory usage monitoring and leak detection
- Custom assertions for domain-specific validation
- Parametrized testing for comprehensive coverage

## Phase 4: CI/CD Integration
- GitHub Actions workflow for automated testing
- Multi-stage testing: unit → integration → e2e → performance
- Code quality checks with flake8, mypy, black, isort
- Security scanning with safety and bandit

## Testing Architecture Benefits
✅ 100+ new test infrastructure components
✅ Standardized test organization (unit/integration/e2e)
✅ Mock-based testing with no external dependencies
✅ Performance regression detection
✅ Comprehensive fixture library
✅ CI/CD pipeline with quality gates

The testing framework supports the domain logic separation and provides
a solid foundation for maintaining high code quality as the system evolves.

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

Co-Authored-By: Claude <noreply@anthropic.com>

domain/issues/exceptions.py

@@ -26,4 +26,19 @@ class IssueStateError(IssueDomainError):
     def __init__(self, message: str, current_state: str, attempted_state: str):
         super().__init__(message)
         self.current_state = current_state
-        self.attempted_state = attempted_state
+        self.attempted_state = attempted_state
+
+
+class IssueNotFoundError(IssueDomainError):
+    """Exception raised when an issue cannot be found."""
+
+    def __init__(self, message: str, issue_number: int = None):
+        super().__init__(message, issue_number)
+
+
+class IssueLabelError(IssueDomainError):
+    """Exception raised when there are label-related issues."""
+
+    def __init__(self, message: str, label_name: str = None):
+        super().__init__(message)
+        self.label_name = label_name