Updated GenerateAuthenticationResultForUserAsync.cs

var tokenDescriptor = new SecurityTokenDescriptor
{
    Subject = new ClaimsIdentity(claims),
    Expires = DateTime.UtcNow.Add(_jwtSettings.TokenLifetime),
    SigningCredentials =
        new SigningCredentials(new SymmetricSecurityKey(key), SecurityAlgorithms.HmacSha256Signature)
};

var token = tokenHandler.CreateToken(tokenDescriptor);

var refreshToken = new RefreshToken
{
    JwtId = token.Id,
    UserId = user.Id,
    CreationDate = DateTime.UtcNow,
    ExpiryDate = DateTime.UtcNow.AddMonths(6)
};

await _context.RefreshTokens.AddAsync(refreshToken);
await _context.SaveChangesAsync();

return new AuthenticationResult
{
    Success = true,
    Token = tokenHandler.WriteToken(token),
    RefreshToken = refreshToken.Token
};