Fix(llm): complete response cache — set_model invalidation, stats logging, sync mutex (#290)

* fix(llm): complete response cache — set_model invalidation, stats logging, sync mutex

# Conflicts:
#	src/llm/response_cache.rs

* fix(llm): address response cache review comments

- Add total_hit_count AtomicU64 that is never decremented on eviction;
  maybe_log_stats now uses this counter so hit_rate_pct stays accurate
  under high eviction pressure
- Log cache stats before returning on provider error so milestone
  intervals (every 100 requests) are never silently skipped
- Add tracing-test dev-dep and three new tests: total_hits_survives_eviction,
  stats_logged_at_request_100, stats_logged_on_provider_error_at_interval
- Update PR description to reflect actual set_model() behavior (key
  isolation, not cache clear)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: nlok5923

Cargo.lock

@@ -174,6 +174,7 @@ zbus = "4"
 
 [dev-dependencies]
 tokio-test = "0.4"
+tracing-test = "0.2"
 tokio-tungstenite = "0.26"
 testcontainers-modules = { version = "0.11", features = ["postgres"] }
 pretty_assertions = "1"