refactor: Make Domain exceptions serializable and add XML docs

- Add [Serializable] attribute to all custom exceptions
- Add protected constructors for serialization support
- Add XML documentation comments
- Update SwissQrCodeNotFoundException message format

DocumentOperator.Domain/Common/Exceptions/PdfProcessingException.cs

@@ -4,6 +4,7 @@
 /// Exception thrown when PDF processing operations fail.
 /// Maps to HTTP 500 Internal Server Error or 422 Unprocessable Entity in the API layer.
 /// </summary>
+[Obsolete("This exception is deprecated. Use more specific exceptions for PDF processing errors.")]
 public class PdfProcessingException : DomainException
 {
     public string Operation { get; }