add tenant-scoped Redis session cache keys and remove dead log_affinity_hit

- Add tenant_header to SessionCacheConfig; when set, cache keys are scoped
  as plano:affinity:{tenant_id}:{session_id} for multi-tenant isolation
- Thread tenant_id through RouterService, routing_service, and llm handlers
- Use Cow<'_, str> in session_key to avoid allocation when no tenant is set
- Remove unused log_affinity_hit (logging was already inlined at call sites)

docs/source/resources/includes/plano_config_full_reference.yaml

@@ -185,6 +185,7 @@ routing:
     type: memory              # "memory" (default) or "redis"
     # url is required when type is "redis". Supports redis:// and rediss:// (TLS).
     # url: redis://localhost:6379
+    # tenant_header: x-org-id  # optional; when set, keys are scoped as plano:affinity:{tenant_id}:{session_id}
 
 # State storage for multi-turn conversation history
 state_storage: