Add testing guidelines

Author: henriquemoody

docs/contributing/development.md

@@ -13,6 +13,13 @@
 - **Unit tests**: Located in `tests/Unit/`
 - **Run all tests**: `composer test`
 
+## Testing Guidelines
+
+1. **Avoid PHPUnit mocks**: Create custom test implementations instead of using createMock()
+2. **Test contracts not implementations**: Verify interactions without depending on specific behavior
+3. **Make test properties public**: When using anonymous classes to access test state
+4. **Verify method calls**: Track whether methods were called and with what parameters
+
 All contributions must include tests that pass.
 
 ## Code Style