Improve friends search and error handling in API

Refactored FriendsController to provide more precise error responses and input validation. Enhanced FriendsService to handle new exception types and improved user search logic. Added tests for user search, updated repository to throw on empty search results, and implemented User equality override. Cleaned up Program.cs controller JSON options.

Govor.Data/Repositories/UsersRepository.cs

@@ -84,7 +84,7 @@ public class UsersRepository : IUsersRepository
                             (f.RequesterId == u.Id && f.AddresseeId == currentUserId)))
             .Take(20)
             .OrderBy(u => u.Username)
-            .ToListAsync();
+            .ToListOrThrowIfEmpty(new NotFoundByKeyException<(string, Guid)>((query, currentUserId), $"Users with given query for user {currentUserId} not found"));
     }