[Enhancement] Adding a constraint in the transformer.

### Search before asking

- [X] I had searched in the [issues](https://github.com/apache/eventmesh/issues?q=is%3Aissue) and found no similar issues.

### Enhancement Request

From a design perspective, the transformer needs to avoid converting non-data fields that users may attempt to transform. The original code allowed users to transform fields in the EventMesh;

### Describe the solution you'd like

Restrictions will be added. If users utilize non-data fields, an exception will be thrown.

### Are you willing to submit PR?

- [X] Yes I am willing to submit a PR!

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct) *

The issue does not name files, tests, or an entry point. Start by locating the transformer implementation and its existing field-handling tests, then define completion as rejecting attempts to transform non-data fields with an exception.

Written by the indexing model from the issue text.