Fix PathUtils.IsSymlink throwing on common lstat failures

[rmarinho]

Summary

IsSymlink previously threw for any lstat failure, including file-not-found and permission-denied. These are expected when walking directory trees (e.g. IsSymlinkOrHasParentSymlink).

Now returns false for ENOENT (2), EACCES (13), and ENOTDIR (20), only throwing for genuinely unexpected errors.

Testing

Added PathUtilsTests with 4 tests verifying behavior for non-existent files, regular files, symlinks, and parent-symlink traversal.

Also adds InternalsVisibleTo for the test assembly.

[Copilot]

Pull request overview

This PR adjusts PathUtils.IsSymlink to treat common lstat failures (e.g., missing path / access denied) as non-exceptional and return false, improving robustness when walking directory trees. It also introduces NUnit coverage for the updated behavior and attempts to expose internals to the test assembly.

Changes:

• Update PathUtils.IsSymlink to return false for specific errno values instead of throwing.
• Add PathUtilsTests to validate symlink detection and non-existent-path behavior.
• Add an InternalsVisibleTo entry intended to allow tests to access internal types.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
Xamarin.MacDev/Xamarin.MacDev.csproj	Adds InternalsVisibleTo for the tests assembly.
Xamarin.MacDev/PathUtils.cs	Updates IsSymlink error handling for lstat failures.
tests/PathUtilsTests.cs	Adds new NUnit tests for PathUtils behavior.