feat: tool function parameters can now include a leading underscore

[cpsievert]

Before this PR, this code would produce a (pydantic) runtime error (now it works)

import chatlas as ctl

chat = ctl.ChatOpenAI()

def add(_x: int, _y: int) -> int:
    return _x + _y

chat.register_tool(add)

[cpsievert]

Errors are due to temporary overloaded server errors -- merging