@cloudflare/pages-plugin-graphql does not return GraphQL Playground

To reproduce, create a new pages project, and create a function with the code shown on the [Cloudflare Pages Docs](https://developers.cloudflare.com/pages/platform/functions/plugins/graphql/#usage).

Note that when running `wrangler pages dev` or having a published project, accessing the GraphQL endpoint results in a 404 instead of the GraphQL playground. However, the usual GraphQL API still works as expected.