fix: UserInputTool prompts now display in Gradio interface

[wingding12]

Summary

Fixes #805

Previously, UserInputTool used Python's input() function directly, which only works in terminal contexts. When used with GradioUI, the prompts would appear in the terminal instead of the Gradio chat interface.

Changes

• UserInputTool now accepts an optional get_user_input callback parameter that can be used to customize how user input is collected
• Default behavior is unchanged (uses terminal input())
• GradioUI automatically configures UserInputTool instances to use a handler that displays questions in the chat interface
• When the agent needs user input, it pauses execution and shows the question in the chat
• The user's next message is treated as the answer to the pending question

New Classes

• GradioUserInputHandler: Manages the input request/response flow between the agent and the Gradio interface
• UserInputRequiredException: Exception raised to pause agent execution when user input is needed

Example Usage

from smolagents import CodeAgent, GradioUI, UserInputTool

agent = CodeAgent(
    tools=[UserInputTool()],  # Works automatically with GradioUI
    model=model
)

# Terminal usage - uses input() as before
tool = UserInputTool()
answer = tool.forward("What is your name?")

# Custom handler
def my_handler(question: str) -> str:
    return get_input_from_my_ui(question)

tool = UserInputTool(get_user_input=my_handler)

Test Plan

• Added tests for UserInputTool with custom input handlers
• All existing GradioUI tests pass
• Verified backward compatibility - default behavior unchanged