Fix seatbelt sandbox symlink handling for rules

[joshvoigts]

Problem: When a file like ~/.gitconfig is a symlink (e.g., pointing into a Dropbox folder), the seatbelt sandbox's path normalization was resolving through symlinks via canonicalize(). This meant the path written into the sandbox policy would be the target path rather than the symlink path the user specified, causing the rule to not match correctly.

Changes:

• Preserve symlinks for allow rules: normalize_path_for_sandbox now uses canonicalize_preserving_symlinks instead of canonicalize(), so symlink paths stay as-is when written into the seatbelt policy. This means a user-specified path like ~/.gitconfig (a symlink) remains that way in the policy and matches correctly.
• Fully resolve symlinks for deny rules: canonicalize_glob_static_prefix_for_sandbox (used for deny rules) continues to use canonicalize() to resolve through symlinks. This ensures the deny regex matches the actual target regardless of how the user refers to it.
• FileSystemAccessMode::None fallback: Fix due to an upstream change.

Additional thoughts
Not sure if it would be worth opening a PR upstream as well...