Add PEP723 header to scripts/validate_solutions.py (#12731)

Enable `uv run scripts/validate_solutions.py` or `pipx run scripts/validate_solutions.py`
* https://peps.python.org/pep-0723
* https://docs.astral.sh/uv/guides/scripts/#declaring-script-dependencies

Author: cclauss

scripts/validate_solutions.py

@@ -1,4 +1,13 @@
 #!/usr/bin/env python3
+
+# /// script
+# requires-python = ">=3.13"
+# dependencies = [
+#     "pytest",
+#     "requests",
+# ]
+# ///
+
 import hashlib
 import importlib.util
 import json