Get Auto Router Benchmarks

Benchmarks for the auto-router dashboard: session shape, savings against the configured baseline, and prompt-caching behaviour bucketed by what the router did.

Reads session rollups folded once per request at spend-write time, so this endpoint never scans LiteLLM_SpendLogs. A user filter selects only turns attributed to that internal user when written; older key-only history remains outside user views. A session is in the window when it overlaps it: its last turn is on or after start_date and its first turn is on or before end_date. Overall hit rate is over telemetry-bearing turns; each bucket's hit rate is over that bucket's turns.

The rollup supplies the measures, never the list. Which routers appear comes from the model registry, so one shows up as soon as it is configured and reads zero until it serves traffic, and routers_in_scope counts those too rather than only the routers the window recorded.