prompt_cache_key was falling back to provider.id when the client did not
supply a session, which collapsed every conversation onto a single key
and defeated upstream prefix caching. Only emit the key when a real
client-provided session/thread identity is available; otherwise let the
upstream use its default matching behaviour.
Additional fixes that affect cache stability:
- Canonicalise (sort) JSON keys in outgoing request bodies and in
tool_call arguments / tool_result content so semantically identical
requests produce identical byte sequences for upstream prefix caches.
- Exempt JSON Schema property maps (properties, patternProperties,
definitions, \$defs) from the underscore-prefix filter so user-defined
schema keys like _id and _meta survive.
- Add a [CacheTrace] debug log with stable hashes for instructions,
tools, input and include to help diagnose cache misses.
- Thread session_id into the usage logger for request correlation.
Jason