[Security] Harden relative path sanitization

[gonzaloriestra]

WHY are these changes introduced?

Improves path sanitization to prevent potential absolute path escapes and directory traversal in commands that use sanitizeRelativePath.

WHAT is this pull request doing?

Hardens sanitizeRelativePath to strip leading slashes (Unix absolute paths), multiple slashes, and Windows drive letters. This ensures that when the sanitized path is joined with a base directory, it cannot escape to arbitrary locations on the filesystem. Added comprehensive unit tests to verify these new sanitization rules.

How to test your changes?

CI

Post-release steps

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes
• I've considered analytics changes to measure impact
• The change is user-facing — I've identified the correct bump type (`patch` for bug fixes · `minor` for new features · `major` for breaking changes) and added a changeset with `pnpm changeset add`

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PR created automatically by Jules for task 6026173477022661726 started by @gonzaloriestra

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