- Added the `ConversionWithSameMimeTypeException` to handle cases where document conversion is requested for the same MIME type.
- Updated `StoredObjectToPdfConverter` to throw the new exception when encountering such cases.
- Enhanced error logging in `PostSendExternalMessageHandler` to capture these specific conversion errors.
- create an event subscriber to catch the workflow which arrive to a "sentExternal" step;
- add a messenger's message to handle the generation of the email;
- add a simple message, and a simple controller for viewing the document
- add dedicated tests
