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)

config/plano_config_schema.yaml

@@ -454,6 +454,11 @@ properties:
           url:
             type: string
             description: Redis URL, e.g. redis://localhost:6379. Required when type is redis.
+          tenant_header:
+            type: string
+            description: >
+              Optional HTTP header name whose value is used as a tenant prefix in the cache key.
+              When set, keys are scoped as plano:affinity:{tenant_id}:{session_id}.
         additionalProperties: false
     additionalProperties: false
   state_storage: