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.API.Tests/IntegrationTests/EF/Repositories/UsersRepositoryTests.cs

@@ -200,7 +200,41 @@ public class UsersRepositoryTests
         Assert.ThrowsAsync<NotFoundByKeyException<IEnumerable<string>>>(async () =>
             await userRepository.FindByRangeUsernamesAsync(usernames));
     }
-
+    
+    [Test]
+    public async Task Given_ValidQuery_When_SearchPotentialFriendsAsync_Then_Returns_Users()
+    {
+        // Arrange
+        var random = new Random();
+        var users = _fixture.CreateMany<User>(random.Next(3, 10)).ToList();
+        
+        await using var context = new GovorDbContext(_options);
+        var userRepository = new UsersRepository(context, _userValidator);
+        
+        context.Users.AddRange(users);
+        await context.SaveChangesAsync();
+        
+        // Act 
+        var result = await userRepository.SearchPotentialFriendsAsync(users[1].Id, users[0].Username[..15]);
+        
+        // Assert 
+        Assert.That(result, Is.Not.Null);
+        Assert.That(result.Count, Is.EqualTo(1));
+        Assert.That(result.First().Id, Is.EqualTo(users[0].Id));
+    }
+    
+    [Test]
+    public async Task Given_InvalidQuery_When_SearchPotentialFriendsAsync_Should_Throw_NotFoundByKeyException()
+    {
+        // Arrange 
+        await using var context = new GovorDbContext(_options);
+        var userRepository = new UsersRepository(context, _userValidator);
+        
+        // Act & Assert 
+        Assert.ThrowsAsync<NotFoundByKeyException<(string,Guid)>>(async () => await 
+            userRepository.SearchPotentialFriendsAsync(_fixture.Create<Guid>(), _fixture.Create<string>()));
+    }
+    
     [Test]
     public async Task Given_ValidDateOnly_When_FindByCreatedDate_Then_Returns_Users()
     {