Use `click` instead of `argparse` on PydanticAI CLI

[Kludex]

Description

We want to simplify the logic we have around the CLI.

References

No response

[assadyousuf]

Mind if I give this a shot? @Kludex

[samuelcolvin]

Are we sure is this necessary?

[Kludex]

Are we sure is this necessary?

We wanted to add clai serve [--protocol] which would serve an agent as an A2A server, but also default clai to the prompt command. To do this without click means to parse the arguments twice, and on each step decide what to do.

Also, I previously explained this in person to you, and you said you were fine with it...

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That said, the goal here is not really to move to click, is to add the clai serve command, while maintaining clai interactive chat, and keeping the code simple.