Demand Control `@listSize(slicingArguments: "...")` does not allow for nested keys

In recent versions of Demand Control, the runtime supports nested keys. To get there, we must go through compostion first, but using something like that fails:

```
type Query {
search(input: SearchInput!): [Book]
@listSize(slicingArguments: ["input.pagination.first"], requireOneSlicingArgument: true)
}
```

Due to the fact that our composition assumes that `slicingArguments` field must point to a single-named field `"input.pagination.first"` instead of checking the nested keys in the input types.

Start by reproducing the composition failure with the nested `input.pagination.first` slicing argument shown in the issue, then trace how composition validates `slicingArguments` against the input types. Done means composition accepts nested keys while preserving the required slicing-argument behavior, with coverage for the provided schema example.

Written by the indexing model from the issue text.