When bot sends an adaptive card, Microsoft Teams notification sent a "Sent a card" message.
Like shown below:
To send custom summary in place of "Sent a card" edit following values...
var response = MessageFactory.Text(string.Empty);
response.Summary = "showing custom greeeting from the Bot - rather than a card";
await turnContext.SendActivityAsync(response, cancellationToken);
Once we add a summary as shown, it will not show "sent a card" message in teams and even in the notification we'll not see "sent a card", we'll get a summary message.