GraphQL SSE via playground.AltairHandler does not work

For this example https://gqlgen.com/recipes/subscriptions/

### versions
- `go run github.com/99designs/gqlgen version`? v0.17.45
- `go version`? go1.21.7 darwin/amd64

But standalone Altair GraphQL Client work fine.

The issue mentions the playground.AltairHandler and a subscription example from the gqlgen documentation. Start by examining the handler implementation in the gqlgen source code, likely under a handler or playground directory. Reproduce the subscription example locally to see the failure. Compare the SSE behavior between the built-in playground and the standalone Altair client to identify the discrepancy.

Vom Indexierungsmodell aus dem Issue-Text verfasst.