handle canonicalize failure in containerized environments

[venkatesh6114]

Description

Fixes #516

path.canonicalize() can fail with ACCESS_DENIED in containerized or sandboxed environments. This caused MathCAT to
bail out entirely when loading rules or reading files.

Changes

• canonicalize_shim (shim_filesystem.rs): Instead of hard-failing, falls back to returning the path as-is (if
  absolute) or prepending cwd (if relative).
• read_to_string_shim (shim_filesystem.rs): Replaces the hard bail!() on canonicalize failure with a fallback to
  the original path, letting read_to_string attempt the read anyway.
• PreferenceManager::initialize (prefs.rs): Same fallback logic applied to rules_dir canonicalization.