added 1 more test file + fixed a bug in the uninstrument method

Author: haneric00

aws-opentelemetry-distro/src/amazon/opentelemetry/distro/opentelemetry/instrumentation/langchain_v2/__init__.py

@@ -42,6 +42,7 @@ def _uninstrument(self, **kwargs):
         if hasattr(self, "_wrapped"):
             for module, name in self._wrapped:
                 unwrap(module, name)
+        self.handler = None
 
 
 class _BaseCallbackManagerInitWrapper: