This looks correct to me, the tests look sensible.

However; it looks like the same issue applies to object, interface, enum and input object extensions?

Actually... for those, it looks like the {...} should be optional in all cases (extension and original definition), though they appear to be required in this parser definition? It's not immediately obvious to me why schema doesn't follow that same pattern, e.g. if you can have

input Foo @oneof
scalar Bar
extend input Foo {
 int: Int
 bar: Bar
}

why could you not also have:

schema @blah
type Query {
 int: Int
}
extend schema {
 query: Query
}

Perhaps this warrants a spec tweak. That's out of scope for this PR though!

I'm unfamiliar with the parser, so I defer to someone more familiar.