[V1][Structured Output] Clear xgrammar compiler object when engine core shut down to avoid nanobind leaked warning (#16954)

Signed-off-by: shen-shanshan <467638484@qq.com>

vllm/v1/engine/core.py

@@ -253,6 +253,7 @@ class EngineCore:
         return engine_core_outputs
 
     def shutdown(self):
+        self.structured_output_manager.clear_backend()
         if self.model_executor:
             self.model_executor.shutdown()