test(permission): add comprehensive tests for CanRead, CanWrite, and combined permission checks

Add TestCanRead, TestCanWrite, TestCanAccessWithReadPermissions, and
TestWritePermissionCombinations to validate the three-layer permission
system including nil user/meta, sub-path inheritance, user whitelists,
and root-level restrictions.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Lanfei