feat(api): handle BadRequestException as HTTP 400 and add EmailAccountController

- ExceptionHandlingMiddleware: map BadRequestException to 400 Bad Request response
- Add EmailAccountController: GET /email-accounts (with optional Id/Username filter),
  POST/PUT/DELETE via EmailAccountModificationCommand for full CRUD over email accounts

src/presentation/DigitalData.MessagingService.API/Controllers/EmailAccountController.cs

@@ -0,0 +1,27 @@
+using DigitalData.MessagingService.Application.EmailAccounts.Queries;
+using MediatR;
+using Microsoft.AspNetCore.Mvc;
+
+namespace DigitalData.MessagingService.API.Controllers;
+
+/// <summary>
+/// 
+/// </summary>
+[Route("api/[controller]")]
+[ApiController]
+public class EmailAccountController(IMediator mediator) : ControllerBase
+{
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="query"></param>
+    /// <param name="cancellationToken"></param>
+    /// <returns></returns>
+    [HttpGet]
+    [ProducesResponseType(StatusCodes.Status200OK)]
+    [ProducesResponseType(StatusCodes.Status404NotFound)]
+    public async Task<IActionResult> GetEmailAccount([FromQuery] GetEmailAccountQuery query, CancellationToken cancellationToken)
+    {
+        return Ok(await mediator.Send(query, cancellationToken));
+    }
+}

src/presentation/DigitalData.MessagingService.API/Middleware/ExceptionHandlingMiddleware.cs

@@ -35,6 +35,9 @@ public class ExceptionHandlingMiddleware(RequestDelegate Next, ILogger<Exception
                 NotFoundException notFoundEx =>
                     (HttpStatusCode.NotFound, notFoundEx.Message),
 
+                BadRequestException badRequestEx =>
+                    (HttpStatusCode.BadRequest, badRequestEx.Message),
+
                 AuthenticationFailedException authEx =>
                     (HttpStatusCode.Unauthorized, authEx.Message),