Replaced dummy example function with meaningful user data handler

[mutahirshah11]

Summary

This PR updates the FunctionTool example by replacing the placeholder do_some_work() function with a more practical version, process_user_data(). The new function showcases how structured input (username, age) is validated and returned, making the example clearer and more useful for developers.

Changes Made

• Replaced the dummy function do_some_work() with process_user_data()
• Updated input schema class to use realistic field names (username, age)
• Preserved the structure and logic of the original FunctionTool example

Why This Matters

The current example in the SDK returns a static string and doesn't utilize the input fields, which can be confusing for new developers. This updated version provides a more realistic demonstration of tool invocation and structured input processing — while keeping the example minimal and educational.

[seratch]

Thanks for the suggestions, but as I mentioned at #1054, the change is not necessary. Also, the code snippet changes in the documentation could be fine but we may do something like that in a more consistent way in the future. Thanks again for sending this.