Add comprehensive testing module with mocks for dbx interfaces

[Copilot]

This PR adds a new testing module that provides mock implementations of all dbx interfaces, making it significantly easier to unit test dbx-based applications without requiring actual database connections or complex sqlmock setups.

What's New

The testing module includes:

• MockExecutor - Complete mock implementation of dbx.Executor interface
• MockDatabase - Mock implementation of dbx.DatabaseWithContext interface
• MockTransactor - Mock implementation of dbx.Transactor interface for transaction testing
• MockContext - Mock implementation of dbx.Context interface with proper context delegation
• MockResult - Mock implementation of sql.Result interface with configurable return values

Usage Example

Before this change, testing dbx-based code required complex sqlmock setup:

func TestUserService_CreateUser(t *testing.T) {
    // Complex sqlmock setup
    mockDB, mock, err := sqlmock.New()
    require.NoError(t, err)
    defer mockDB.Close()
    
    mock.ExpectExec("INSERT INTO users").
        WithArgs("John").
        WillReturnResult(sqlmock.NewResult(123, 1))
    
    db := dbx.New(mockDB)
    service := NewUserService(db)
    // ... rest of test
}

Now you can use the simpler mock interface:

func TestUserService_CreateUser(t *testing.T) {
    // Simple mock setup
    mockDB := dbxtesting.NewMockDatabase()
    expectedResult := dbxtesting.NewResult(123, 1)
    mockDB.On("Exec", "INSERT INTO users (name) VALUES (?)", "John").
        Return(expectedResult, nil)
    
    service := NewUserService(mockDB)
    userID, err := service.CreateUser(context.Background(), "John")
    
    require.NoError(t, err)
    assert.Equal(t, int64(123), userID)
    mockDB.AssertExpectations(t)
}

Key Features

• testify/mock Integration: Uses the popular testify/mock framework for familiar expectation setup
• Full Interface Compatibility: Works with all dbx interfaces (Executor, Database, Context, etc.)
• Error Testing Support: Easy testing of both operation errors and result-specific errors
• Helper Functions: NewResult(), NewResultWithError() for quick result mocking
• Comprehensive Documentation: Complete README with usage patterns and best practices

Testing

The module includes comprehensive tests covering all mock functionality and real-world usage examples. All existing tests continue to pass, ensuring backward compatibility.

The testing module makes unit testing dbx-based code much more straightforward while maintaining the same interface-driven design principles that make dbx flexible and testable.

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