Subscription event DataFetcherResult.extensions are missing from emitted ExecutionResult #4475

When a subscription publisher emits a DataFetcherResult containing extensions, those extensions are not included in the emitted ExecutionResult.

Reproduction

A subscription data fetcher returns:

return Mono.just(
    DataFetcherResult.newResult()
        .data(event)
        .extensions(Map.of("myKey", "myValue"))
        .build()
);

The emitted result contains the expected data, but:

executionResult.getExtensions() == null

Adding the value directly to the ExtensionsBuilder during the initial data-fetcher invocation causes it to appear on the initial subscription result:

 dfe.getGraphQlContext()
     .get(ExtensionsBuilder.class)
     .addValue("myKey", "myValue");

However, extensions supplied by an emitted DataFetcherResult are missing from the corresponding subscription event result.