add explicit agent end method (#486)

tests/unit_tests/agents/test_agent.py

@@ -90,8 +90,9 @@ def test_agent_with_callbacks() -> None:
     output = agent.run("when was langchain made")
     assert output == "curses foiled again"
 
-    # 1 top level chain run, 2 LLMChain runs, 2 LLM runs, 1 tool run, 1 ending
-    assert handler.starts == 7
+    # 1 top level chain run, 2 LLMChain runs, 2 LLM runs, 1 tool run
+    assert handler.starts == 6
+    # 1 extra agent end
     assert handler.ends == 7
     assert handler.errors == 0
 

tests/unit_tests/callbacks/fake_callback_handler.py

@@ -58,3 +58,7 @@ class FakeCallbackHandler(BaseCallbackHandler):
     def on_tool_error(self, error: Exception) -> None:
         """Run when tool errors."""
         self.errors += 1
+
+    def on_agent_end(self, log: str, **kwargs: Any) -> None:
+        """Run when agent is ending."""
+        self.ends += 1