Fix: Add run_manager on all AgentFinish returns in AgentExecutor

[kaikun213]

Fix: Adds run_manager to all return calls on AgentExecutor

Currently, run_manager is only passed to _return / _areturn if we stop the execution due to timeout/max-iterations. We also would like to have the callback signal for on_agent_finish when the agent finished due to tool-return or simply decides to take that action.

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Project lead @hwchase17

Tracing / Callbacks / Async @agola11