List accumulating failure categories for a metric

Return the persistent failure categories for a project (optionally one metric), each with a call count and example call ids. Filter by date_from / date_to (ISO datetimes for an exact window, or bare dates for whole-day bounds) to count only assignments in that range, and by agent_ids (comma-separated agent ids) or agent_version_ids (comma-separated agent version ids) to count only calls handled by those agents / agent versions; results are sorted by count descending. Each category also carries agents: the agents that handled its calls, busiest first, each with a call count, its own example call ids, and whether Improve Agent can edit it. And agent_versions: the agent versions those calls ran on, NEWEST VERSION FIRST, each with the version's name, when it went live, a call count, and its own example call ids — read it to tell whether a version introduced or fixed the failure mode.