[BUG] MultiStepAgent silently overwrites parameter-provided FinalAnswerTool

[crcdng]

Describe the bug
MultiStepAgent silently overwrites the parameter-provided FinalAnswerTool.

The agent receives a list of tools trough its constructor, which may include a 'FinalAnswerTool' that terminates the Thought-Action-Observation cycle of the agent.

However agent.py, in line 255 silently overrides a user-provided FinalAnswerTool by instantiating the default implementation.

self.tools["final_answer"] = FinalAnswerTool()

This behavior violates the Principle of Least Surprise and can lead to hard to track bugs. It also creates a hard dependency on the library-povided FinalAnswerTool class and makes it impossible to provide a modified FinalAnswerTool by a user of the API.

Release v. 1.9.2

Additional context
Originally reported by user Julien in the huggingface Discord.

[Knight7561]

I guess #769 would fix this?

[albertvillanova]

Thanks for raising this issue, @crcdng.

And thanks @Knight7561 for pointing to #769 PR.

I guess that PR partially solves the issue, because it allows to customize "final_answer" after instantiation. Maybe we should also support customize "final_answer" at instantiation, as we already do with the rest of the tools.