Optional trace policy for this middleware's hook spans (wrap_model_call/
wrap_tool_call and the before_*/after_* node hooks).
By default (None), hook spans are traced normally. Set a TracePolicy to shape
what they record -- e.g. TracePolicy(process_inputs=omit_payload) to drop the
conversation messages/state payload while keeping the span and its timing.
Messages are still captured on the inner model-call span.
