[T-075] A truncated figure description is stored in the shared cross-request cache as if complete

[Ztein]

Canonical ticket: docs/tickets/T-075-truncated-descriptions-shared-as-complete.md

A finish_reason=length description is warned about on the generating request (T-033) but written to the shared cross-request cache, where later documents reuse it silently for the full TTL. Measured by scorecard row Q1 (scripts/cache_bench/bench.py).