Return access_denied error when an OAuth2 request is denied (#30974) (#31029)

Backport #30974 by Zettat123

Co-authored-by: Zettat123 <zettat123@gmail.com>
Co-authored-by: KN4CK3R <admin@oldschoolhack.me>

routers/web/auth/oauth.go

@@ -541,6 +541,16 @@ func GrantApplicationOAuth(ctx *context.Context) {
 		ctx.Error(http.StatusBadRequest)
 		return
 	}
+
+	if !form.Granted {
+		handleAuthorizeError(ctx, AuthorizeError{
+			State:            form.State,
+			ErrorDescription: "the request is denied",
+			ErrorCode:        ErrorCodeAccessDenied,
+		}, form.RedirectURI)
+		return
+	}
+
 	app, err := auth.GetOAuth2ApplicationByClientID(ctx, form.ClientID)
 	if err != nil {
 		ctx.ServerError("GetOAuth2ApplicationByClientID", err)