feat(domain): add domain exception hierarchy

- DomainException: Base exception for all domain errors
- ValidationException: Business rule validation failures
- AttachmentProcessingException: Attachment-specific processing errors

Exceptions maintain error code compatibility with legacy system.

src/DigitalData.EmailProfiler.Domain/Exceptions/AttachmentProcessingException.cs

@@ -0,0 +1,19 @@
+using DigitalData.EmailProfiler.Domain.Enums;
+
+namespace DigitalData.EmailProfiler.Domain.Exceptions;
+
+public class AttachmentProcessingException : DomainException
+{
+    public ErrorCode ErrorCode { get; }
+
+    public AttachmentProcessingException(ErrorCode errorCode, string message) : base(message)
+    {
+        ErrorCode = errorCode;
+    }
+
+    public AttachmentProcessingException(ErrorCode errorCode, string message, Exception innerException) 
+        : base(message, innerException)
+    {
+        ErrorCode = errorCode;
+    }
+}

src/DigitalData.EmailProfiler.Domain/Exceptions/DomainException.cs

@@ -0,0 +1,12 @@
+namespace DigitalData.EmailProfiler.Domain.Exceptions;
+
+public class DomainException : Exception
+{
+    public DomainException(string message) : base(message)
+    {
+    }
+
+    public DomainException(string message, Exception innerException) : base(message, innerException)
+    {
+    }
+}

src/DigitalData.EmailProfiler.Domain/Exceptions/ValidationException.cs

@@ -0,0 +1,12 @@
+namespace DigitalData.EmailProfiler.Domain.Exceptions;
+
+public class ValidationException : DomainException
+{
+    public ValidationException(string message) : base(message)
+    {
+    }
+
+    public ValidationException(string message, Exception innerException) : base(message, innerException)
+    {
+    }
+}