Break up long steps arrays

Fetching too many deltas with >8 steps per delta on average can cause issues returning arrays that are too big (8k length limit). Return fewer items per array - potentially by just returning the deltas instead.

Trace the code path that fetches deltas and returns step arrays; the issue gives no file or test entry point, so start by searching for the delta-fetching response. Reproduce a case averaging more than eight steps per delta and verify that returned arrays stay below the 8k length limit, either by returning fewer items or deltas.

Written by the indexing model from the issue text.