Fix bug on notification read (#36339) (#36387)

Backport #36339 by @lunny

When a user has been revoked permission to access a repository, the
related notification could still be visited. But the repository's
information should not be leaked any more.

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

services/convert/notification_test.go

@@ -0,0 +1,57 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package convert
+
+import (
+	"testing"
+
+	activities_model "code.gitea.io/gitea/models/activities"
+	repo_model "code.gitea.io/gitea/models/repo"
+	"code.gitea.io/gitea/models/unittest"
+	user_model "code.gitea.io/gitea/models/user"
+	"code.gitea.io/gitea/modules/timeutil"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestToNotificationThreadIncludesRepoForAccessibleUser(t *testing.T) {
+	assert.NoError(t, unittest.PrepareTestDatabase())
+
+	n := newRepoNotification(t, 1, 4)
+	thread := ToNotificationThread(t.Context(), n)
+
+	if assert.NotNil(t, thread.Repository) {
+		assert.Equal(t, n.Repository.FullName(), thread.Repository.FullName)
+		assert.Nil(t, thread.Repository.Permissions)
+	}
+}
+
+func TestToNotificationThreadOmitsRepoWhenAccessRevoked(t *testing.T) {
+	assert.NoError(t, unittest.PrepareTestDatabase())
+
+	n := newRepoNotification(t, 2, 4)
+	thread := ToNotificationThread(t.Context(), n)
+
+	assert.Nil(t, thread.Repository)
+}
+
+func newRepoNotification(t *testing.T, repoID, userID int64) *activities_model.Notification {
+	t.Helper()
+
+	ctx := t.Context()
+	repo := unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{ID: repoID})
+	assert.NoError(t, repo.LoadOwner(ctx))
+	user := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: userID})
+
+	return &activities_model.Notification{
+		ID:          repoID*1000 + userID,
+		UserID:      user.ID,
+		RepoID:      repo.ID,
+		Status:      activities_model.NotificationStatusUnread,
+		Source:      activities_model.NotificationSourceRepository,
+		UpdatedUnix: timeutil.TimeStampNow(),
+		Repository:  repo,
+		User:        user,
+	}
+}