Add FetchEmailByUid endpoint and improve middleware

Added a new endpoint to `EmailController` for fetching a single email by UID, including route and query parameter handling, and appropriate Swagger annotations.

Refactored `ExceptionHandlingMiddleware` to use constructor injection, simplified exception handling with a `switch` expression, and improved logging and response formatting. Added support for `ValidationException`.

Updated `FormatValidationErrors` to handle `ValidationException` and improved error formatting.

Made minor documentation updates in `EmailController` and performed general code cleanup for readability and modernization.

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

@@ -72,6 +72,7 @@ public class EmailController(IMediator mediator) : ControllerBase
     /// Fetch emails from an IMAP mailbox.
     /// </summary>
     /// <param name="query">Query parameters for filtering and fetching emails from the IMAP mailbox.</param>
+    /// <param name="firstHtmlBodyOnly"></param>
     /// <param name="cancellationToken">Cancellation token.</param>
     /// <returns>HTTP 200 with list of received emails.</returns>
     [HttpGet]
@@ -91,6 +92,28 @@ public class EmailController(IMediator mediator) : ControllerBase
         return Ok(emails);
     }
 
+    /// <summary>
+    /// Fetch a single email by its UID from an IMAP mailbox.
+    /// </summary>
+    /// <param name="uid">The unique identifier (UID) of the message.</param>
+    /// <param name="query">Query parameters including account, folder and attachment flag.</param>
+    /// <param name="cancellationToken">Cancellation token.</param>
+    /// <returns>HTTP 200 with the matched email, or HTTP 404 if not found.</returns>
+    [HttpGet("{uid:long}")]
+    [ProducesResponseType(StatusCodes.Status200OK)]
+    [ProducesResponseType(StatusCodes.Status400BadRequest)]
+    [ProducesResponseType(StatusCodes.Status404NotFound)]
+    public async Task<IActionResult> FetchEmailByUid([FromRoute] long uid, [FromQuery] FetchEmailByUidQuery query, CancellationToken cancellationToken = default)
+    {
+        var queryWithUid = query.WithUid(uid);
+        var email = await mediator.Send(queryWithUid, cancellationToken);
+
+        if (email is null)
+            return NotFound($"No email found with UID {uid}.");
+
+        return Ok(email);
+    }
+
     /// <summary>
     /// Mark a single IMAP message as seen (read).
     /// </summary>