The call-block decoration reads agent._use_prompt_caching / _cache_ttl /
_use_native_cache_layout directly; the redecoration helper wrapped each in
getattr with divergent defaults (e.g. or-'5m' vs verbatim _cache_ttl).
The flags are unconditionally initialized on AIAgent, so the defaults
served only test fixtures and would mask a real init bug as silent
cache-off. Align with the house style.