Make RecentReceiverSuggestionDto immutable; refactor query

Updated RecentReceiverSuggestionDto to use `init` properties,
adding immutability and a new `LastUsedAt` field. Refactored
ReadRecentReceiverSuggestionsQueryHandler to simplify query
logic, remove `IMapper` dependency, and improve determinism
in results. Introduced tie-breaking for duplicate timestamps
and optimized ordering by `LastUsedAt` and `EmailAddress`.
Removed redundant `Include` and mapping steps for clarity.

EnvelopeGenerator.Application/Common/Dto/Receiver/RecentReceiverSuggestionDto.cs

@@ -2,7 +2,9 @@ namespace EnvelopeGenerator.Application.Common.Dto.Receiver;
 
 public class RecentReceiverSuggestionDto
 {
-    public required string FullName { get; set; }
+    public required string FullName { get; init; }
 
-    public required string EmailAddress { get; set; }
+    public required string EmailAddress { get; init; }
+
+    public required DateTime LastUsedAt { get; init; }
 }