fix errcheck in server/channels/app/platform/session.go (#30595)

Co-authored-by: Ben Schumacher <ben.schumacher@mattermost.com>

server/channels/app/oauth.go

@@ -403,7 +403,9 @@ func (a *App) newSession(c request.CTX, app *model.OAuthApp, user *model.User) (
 		return nil, model.NewAppError("newSession", "api.oauth.get_access_token.internal_session.app_error", nil, "", http.StatusInternalServerError).Wrap(err)
 	}
 
-	a.ch.srv.platform.AddSessionToCache(session)
+	if err := a.ch.srv.platform.AddSessionToCache(session); err != nil {
+		c.Logger().Warn("Failed to add session to cache", mlog.Err(err))
+	}
 
 	return session, nil
 }