Add .md extension to Arc registration troubleshooting guide

[Copilot]

This PR adds the missing .md extension to the file TSG/ArcRegistration/TSG | Arc registration failing with error 42.

The file has been renamed to TSG | Arc registration failing with error 42.md to properly identify it as a markdown document, which matches the convention used by other troubleshooting guides in the repository. This makes the file:

• More recognizable as a markdown document
• Consistent with other documentation files
• Compatible with tools that expect markdown files to have the .md extension
• Properly rendered with markdown formatting in browsers and editors

No changes were made to the file's contents or location.

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