fix(demos): Fix Splunk integration in Corelight notebook

[SinsBre]

Previous implementation had broken JSON parsing in splunkToPandas function causing data retrieval failures.

• Fix splunkToPandas to use JSONResultsReader with proper output_mode
• Fix regex escape sequence warnings in helper functions
• Add proper error handling for pagination
• Clean up notebook structure

This ensures the demo notebook works correctly with current Splunk SDK and maintains clean example code.