fix: improve code quality for SonarCloud compliance

- Reverted to more maintainable loop-based approach for field copying
- Ensures type safety while addressing SonarCloud code quality concerns
- Maintains functionality for preserving examples when using model field
- Addresses SonarCloud finding identified by dreamorosi

Author: dcabib

aws_lambda_powertools/event_handler/api_gateway.py

@@ -682,7 +682,6 @@ def _get_openapi_path(  # noqa PLR0912
 
                             # Preserve existing fields like examples, encoding, etc.
                             new_payload: OpenAPIResponseContentSchema = {}
-                            # Copy all fields except 'model'
                             for key, value in payload.items():
                                 if key != "model":
                                     new_payload[key] = value  # type: ignore[literal-required]