Refactor user context handling with CurrentUserService

Introduced ICurrentUserService and its implementation to centralize retrieval of the current user's ID from claims. Updated FriendsController to use this service instead of direct claim access. Registered the service and IHttpContextAccessor in DI. Added integration tests for FriendsController. Moved UsernameValidator to Infrastructure/Validators.

Govor.Application/Infrastructure/Validators/UsernameValidator.cs

@@ -0,0 +1,35 @@
+using System.Linq.Expressions;
+using System.Text.RegularExpressions;
+using Govor.Application.Exceptions.AuthService;
+using Govor.Application.Interfaces.Authentication;
+using Govor.Core.Infrastructure.Validators;
+
+namespace Govor.Application.Validators;
+
+public class UsernameValidator : IUsernameValidator
+{
+    private readonly Regex _usernameRegex = new(@"^[А-Яа-яЁё]+$", RegexOptions.Compiled);
+    
+    public void Validate(string username)
+    {
+        if(username.Length < UserValidator.MIN_LENGHT_OF_NAME || username.Length > UserValidator.MAX_LENGHT_OF_NAME)
+            throw new InvalidUsernameException($"Username must be between {UserValidator.MIN_LENGHT_OF_NAME} and {UserValidator.MAX_LENGHT_OF_NAME} characters.");
+
+        if (!_usernameRegex.IsMatch(username))
+            throw new InvalidUsernameException("The username must be in Cyrillic and start with a letter.");
+    }
+
+    public bool TryValidate(string username)
+    {
+        try
+        {
+            Validate(username);
+            return true;
+        }
+        catch
+        {
+            return false;
+        }
+    }
+
+}