fix(reach): do not cache results of interrupted queries (port of upstream PR 2277) #593

Port and adapt the upstream fix (Kpa-clawbot/CoreScope pull request number 2277, merged 2026-10-25; literal reference, intentionally not linked). Reach may cache a result from a query that was interrupted (context cancelled, timeout, admission reject), so later requests are served an incomplete or empty report.

Check our Reach implementation (admission limits #566, the 429 retry #547 and singleflight) for the same bug. Fix it so only complete results are cached, with a test that reproduces the bug (red before, green after). Preserve fork-specific behaviour.