After a partial update (stash restore overwriting providers/base.py with
an older version), the NousProfile singleton was instantiated from a
ProviderProfile class that predates the supports_prompt_cache_key field
(added in f4fb23f3d). Accessing profile.supports_prompt_cache_key raised
AttributeError, crashing every API call with:
'NousProfile' object has no attribute 'supports_prompt_cache_key'
Use getattr(profile, 'supports_prompt_cache_key', False) so a stale
profile degrades to 'no prompt cache key' instead of crashing.
