[WIP]changing file default value in long context demo

[przepeck]

🛠 Summary

CVS-179934
Changing default value for script which is used to create dataset for demo

🧪 Checklist

• Unit tests added.
• The documentation updated.
• Change follows security best practices.
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[Copilot]

Pull request overview

This PR updates the default file URL in the long context demo dataset creation script to use a different source from Project Gutenberg instead of the Bodleian library.

Changes:

• Changed the default --file_url argument value to point to a different text file source

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[mzegla]

Please update also: https://github.com/openvinotoolkit/model_server/tree/przepeck/long_ctx_demo_fix/demos/continuous_batching/long_context#dataset-for-experiments
Rerun those steps with new input and update required sections.