Add Python example for thread-safe Define using rdfslot_

tutorials/analysis/dataframe/df_python_threadsafe_example.py

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+## \file
+## \ingroup tutorial_dataframe
+## \notebook -nodraw
+##
+## Demonstrates a thread-safe column definition in Python using `rdfslot_`.
+##
+## In C++, ROOT provides `DefineSlot` and `RedefineSlot` for thread-safe operations.
+## However, these APIs are not currently available in Python.
+##
+## This example shows how to achieve similar behavior using `rdfslot_`,
+## which represents the slot (thread) ID assigned during execution.
+##
+## This approach ensures that each thread works with its own data,
+## avoiding race conditions in multi-threaded environments.
+import ROOT
+def main():
+    # Enable implicit multi-threading
+    ROOT.ROOT.EnableImplicitMT()
+    print("Multithreading Enabled:", ROOT.ROOT.IsImplicitMTEnabled())
+    # Create a simple RDataFrame with 20 entries
+    df = ROOT.RDataFrame(20)
+    # Define thread-safe columns using rdfslot_
+    df = (
+        df.Define("thread_id", "rdfslot_")
+          .Define("value", "rdfslot_ * 10")
+          .Define("combined", "thread_id + value")
+    )
+    print("\nDisplaying DataFrame with thread-aware columns:\n")
+    # Display results
+    df.Display(["thread_id", "value", "combined"]).Print()
+    print("\nExplanation:")
+    print("- 'thread_id' shows which thread processed each entry.")
+    print("- 'value' is derived independently per thread.")
+    print("- 'combined' demonstrates safe computation using thread-local values.")
+if __name__ == "__main__":
+    main()