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/DomainException.cs

@@ -4,6 +4,7 @@
 /// Base exception for all domain-related exceptions.
 /// Caught by the Exception Handling Middleware in the API layer.
 /// </summary>
+[Obsolete("This exception is deprecated. Use more specific exceptions for domain errors.")]
 public abstract class DomainException : Exception
 {
     /// <summary>

DocumentOperator.Domain/Common/Exceptions/DomainValidationException.cs

@@ -4,6 +4,7 @@
 /// Exception thrown when domain validation fails (e.g., invalid Value Objects).
 /// Maps to HTTP 400 Bad Request in the API layer.
 /// </summary>
+[Obsolete("This exception is deprecated. Use more specific exceptions for domain validation errors.")]
 public class DomainValidationException : DomainException
 {
     public string PropertyName { get; }

DocumentOperator.Domain/Common/Exceptions/NotFoundException.cs

@@ -1,25 +1,22 @@
-namespace DocumentOperator.Domain.Common.Exceptions;
+using System.Runtime.Serialization;
+
+namespace DocumentOperator.Domain.Common.Exceptions;
 
 /// <summary>
 /// Exception thrown when a requested resource is not found.
 /// Maps to HTTP 404 Not Found in the API layer.
 /// </summary>
-public class NotFoundException : DomainException
+public class NotFoundException : Exception
 {
-    public string ResourceType { get; }
-    public object ResourceId { get; }
-
-    public NotFoundException(string resourceType, object resourceId)
-        : base($"{resourceType} with ID '{resourceId}' was not found.", "RESOURCE_NOT_FOUND")
+    public NotFoundException()
     {
-        ResourceType = resourceType;
-        ResourceId = resourceId;
     }
 
-    public NotFoundException(string resourceType, object resourceId, string customMessage)
-        : base(customMessage, "RESOURCE_NOT_FOUND")
+    public NotFoundException(string? message) : base(message)
+    {
+    }
+
+    public NotFoundException(string? message, Exception? innerException) : base(message, innerException)
     {
-        ResourceType = resourceType;
-        ResourceId = resourceId;
     }
 }

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; }