ResultPager fetchall on apps/installations incorrect result

See https://platform.github.community/t/pagination-differs-in-installations-api/1862

The result is an array with a count field and and subarray of all installations but we merge result when using the fetchAll method

We need to check if this is something we need/can fix in code or if we should add some docs about this issue

Start by tracing ResultPager's fetchAll handling for the apps/installations endpoint and compare it with the endpoint's documented array, count, and installations structure. Determine whether the client can return that structure correctly or whether the behavior needs documentation; done means resolving that choice and covering the resulting behavior or guidance.

索引モデルが issue の本文から書いたものです。