Refactor email query and controller endpoints

Refactored `FetchEmailsQuery` to encapsulate filtering and retrieval parameters (`UnseenOnly` and `MaxCount`) within a nested `MailQuery` record for better organization. Updated `FetchEmailsQueryHandler` to use the new structure.

Simplified `EmailController` endpoints:
- Replaced multiple query parameters in `FetchEmails` with a single `FetchEmailsQuery` object.
- Replaced multiple parameters in `MarkAsSeen` with a `MarkEmailAsSeenCommand` object.
- Adjusted the HTTP route for `MarkAsSeen` from `"{uid}/seen"` to `"seen"`.

Updated XML documentation to reflect these changes, improving maintainability and scalability.

src/core/DigitalData.MessagingService.Application/EmailReceiving/Queries/FetchEmailsQuery.cs

@@ -22,14 +22,22 @@ public record FetchEmailsQuery : IRequest<IEnumerable<ReceivedEmailContext>>
     public string Folder { get; init; } = "INBOX";
 
     /// <summary>
-    /// When <see langword="true"/> returns only unread messages.
+    /// Mail query used to filter and limit the emails retrieved.
     /// </summary>
-    public bool UnseenOnly { get; init; } = false;
+    public MailQuery Mail { get; init; } = new();
 
-    /// <summary>
-    /// Maximum number of messages to retrieve (most-recent first). 0 = unlimited.
-    /// </summary>
-    public int MaxCount { get; init; } = 50;
+    public record MailQuery
+    {
+        /// <summary>
+        /// When <see langword="true"/> returns only unread messages.
+        /// </summary>
+        public bool UnseenOnly { get; init; } = false;
+
+        /// <summary>
+        /// Maximum number of messages to retrieve (most-recent first). 0 = unlimited.
+        /// </summary>
+        public int MaxCount { get; init; } = 50;
+    }
 }
 
 public class FetchEmailsQueryHandler(
@@ -49,8 +57,8 @@ public class FetchEmailsQueryHandler(
         return await ImapService.FetchEmailsAsync(
             account,
             request.Folder,
-            request.UnseenOnly,
-            request.MaxCount,
+            request.Mail.UnseenOnly,
+            request.Mail.MaxCount,
             cancellationToken);
     }
 }