[Security] Prevent absolute path bypass in sanitizeRelativePath

[gonzaloriestra]

WHY are these changes introduced?

The sanitizeRelativePath function is intended to ensure a path provided by a user (e.g., in a configuration file) is relative and does not escape the intended directory. Previously, it only checked for .. segments but allowed absolute paths (starting with / or Windows drive letters like C:\) to pass through. This could allow an attacker to overwrite sensitive system files if the path is used as a destination in a file operation.

WHAT is this pull request doing?

• Enhanced sanitizeRelativePath to detect and strip leading forward slashes and Windows drive letters.
• Updated the function to filter out empty segments and drive letter segments.
• Improved the warning message to correctly reflect that the path was insecure (either due to traversal or being absolute).
• Added comprehensive unit tests in packages/cli-kit/src/public/node/path.test.ts to verify the fix across different OS path styles.

How to test your changes?

You can verify the behavior by calling the sanitizeRelativePath function with various inputs in a Node environment or via unit tests.

Duplicate check

• Query: gh pr list --repo Shopify/cli --state all --limit 200 --search 'label:"Jules" "Security" in:title' --json number,title,state,url,body,files
• Closest PR: [Security] Redact cookies in sanitized headers output (Different subsystem, unrelated to path sanitization).

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've added unit tests.

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PR created automatically by Jules for task 7639287056760175654 started by @gonzaloriestra

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