Add Python example for thread-safe Define using rdfslot_

[gayatripadalia]

Summary

This PR adds a Python tutorial demonstrating a thread-safe pattern
using rdfslot_ in ROOT RDataFrame.

In C++, ROOT provides DefineSlot and RedefineSlot for thread-safe
operations. However, these APIs are currently not available in Python.

This example shows how rdfslot_ can be used as a practical and safe
workaround to achieve thread-local computation in a multi-threaded
environment.

This contribution serves as a practical Python-side alternative until native support becomes available.

Motivation

Issue #20839 highlights the lack of Python examples for thread-safe operations similar to DefineSlot.

This contribution aims to:

• Provide a clear and minimal Python example
• Help users understand slot-based computation
• Offer a practical workaround using existing features

Changes Made

• Added new tutorial:
  tutorials/analysis/dataframe/df_python_threadsafe_example.py
• Demonstrates:
  • Enabling implicit multi-threading
  • Using rdfslot_ to identify thread/slot
  • Performing safe, thread-local computations
• Includes inline explanations for beginner accessibility

Testing

• The script runs successfully with ROOT Python bindings
• Output verifies correct usage of rdfslot_ across threads
• No external dependencies required

Notes for Reviewers

• The example is intentionally simple and educational
• Designed to be beginner-friendly while demonstrating correct usage
• Does not attempt to reimplement DefineSlot, but provides a practical alternative

Related Issue

Closes #20839