[AI assisted] MM-62837: (#30268)

We did not invalidate the cache after converting a user to bot.
That led to issues. See JIRA for more details.

https://mattermost.atlassian.net/browse/MM-62837

```release-note
NONE
```

server/channels/api4/user_test.go

@@ -6601,6 +6601,25 @@ func TestConvertUserToBot(t *testing.T) {
 		require.NoError(t, err)
 		require.NotNil(t, bot)
 	})
+
+	t.Run("user cannot login after being converted to bot", func(t *testing.T) {
+		// Create a new user
+		user := th.CreateUser()
+
+		// Login as the new user to verify login works initially
+		_, _, err := th.Client.Login(context.Background(), user.Email, user.Password)
+		require.NoError(t, err)
+
+		// Convert user to bot
+		_, _, err = th.SystemAdminClient.ConvertUserToBot(context.Background(), user.Id)
+		require.NoError(t, err)
+
+		// Try to login again - should fail
+		_, resp, err := th.Client.Login(context.Background(), user.Email, user.Password)
+		require.Error(t, err)
+		CheckErrorID(t, err, "api.user.login.bot_login_forbidden.app_error")
+		CheckUnauthorizedStatus(t, resp)
+	})
 }
 
 func TestGetChannelMembersWithTeamData(t *testing.T) {

server/channels/app/bot.go

@@ -617,6 +617,7 @@ func (a *App) ConvertUserToBot(rctx request.CTX, user *model.User) (*model.Bot,
 	if err := a.RevokeAllSessions(rctx, user.Id); err != nil {
 		return nil, err
 	}
+	a.InvalidateCacheForUser(user.Id)
 
 	return bot, nil
 }