Cleanup unused directives and add BadRequestException

Removed unnecessary using directives in `UpdateEmailTemplateCommandHandler.cs`. Added a new `BadRequestException` class in `BadRequestException.cs` to handle bad request scenarios with customizable error messages.

EnvelopeGenerator.Application/EmailTemplates/Commands/Update/UpdateEmailTemplateCommandHandler.cs

@@ -3,8 +3,6 @@ using EnvelopeGenerator.Application.DTOs;
 using EnvelopeGenerator.Application.Exceptions;
 using EnvelopeGenerator.Domain.Entities;
 using MediatR;
-using Microsoft.AspNetCore.Mvc;
-using Microsoft.EntityFrameworkCore;
 
 namespace EnvelopeGenerator.Application.EmailTemplates.Commands.Update;
 

EnvelopeGenerator.Application/Exceptions/BadRequestException.cs

@@ -0,0 +1,22 @@
+namespace EnvelopeGenerator.Application.Exceptions;
+
+/// <summary>
+/// Represents an exception that is thrown when a bad request is encountered.
+/// </summary>
+public class BadRequestException : Exception
+{
+    /// <summary>
+    /// Initializes a new instance of the <see cref="BadRequestException"/> class.
+    /// </summary>
+    public BadRequestException()
+    {
+    }
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="BadRequestException"/> class with a specified error message.
+    /// </summary>
+    /// <param name="message">The message that describes the error.</param>
+    public BadRequestException(string? message) : base(message)
+    {
+    }
+}