Codegen should take @include and @skip into account

If we use `@include` or `@skip` directives on a non-nullable field, the code generation output should be nullable because we can guarantee that the data won't always exist.

No files or tests are named; start at the code-generation entry point for GraphQL fields and trace handling of @include and @skip on non-nullable fields. Done means generated output marks fields using these directives as nullable while preserving existing behavior for fields without them, with regression coverage.

Written by the indexing model from the issue text.