Apollo codegen query results should not be optional for pure @client queries

If all fields in the query are `@client`, then the top-level `data` will always be defined!

Start by locating the Apollo codegen handling for queries whose fields are all marked @client, then inspect the generated query result type. Verify how the top-level data field is typed for a pure client query; done means it is non-optional while existing behavior for other queries remains covered.

Written by the indexing model from the issue text.