Improve unique directory generation for temp files

[ericstj]

Refactor unique directory creation logic to use random file names.

Fixes #7485

[Copilot]

Pull Request Overview

This PR refactors the unique directory creation logic for temporary files to use random file names instead of incrementing directory numbers. The change replaces a counter-based approach with a random filename approach to generate unique temporary directory names.

• Replaces incremental directory numbering with random filename generation
• Simplifies the directory creation loop logic
• Updates the directory naming pattern from ml_dotnet{number} to ml_dotnet_{randomfilename}

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