refactor the logic in agent_controller to imporve readability

[WannaTen]

Short description of the problem this fixes or functionality that this introduces. This may be used for the CHANGELOG

mainly on refactor the logic in agent_controller to imporve readability.

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Give a summary of what the PR does, explaining any non-trivial design decisions

• on_event: Refactored event processing by splitting it into two distinct functions: _handle_action and _handle_observation
• set_agent_state_to: Consolidated new_state checks into a single if..elif structure, replacing multiple parallel if statements
• _step: Isolated delegate step functionality and integrated traffic limit handling logic

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Link of any specific issues this addresses

[WannaTen]

I'm still working on this. modifying testcases to validate the entire set of modifications. Besides, maybe the "confirmed by human" step can be improved futher,

[neubig]

Thanks a lot for this! This already looks like a significant improvement over what we had before. Really appreciated.