fix: restore accidentally removed test suites in CustomModesManager

- Restored updateCustomMode test suite
- Restored deleteCustomMode test suite
- Restored File Operations test suite
- Restored importModeWithRules test suite
- Restored checkRulesDirectoryHasContent test suite
- Restored exportModeWithRules test suite

These test suites were accidentally removed in commit b518a48 during refactoring.
All 51 tests are now passing successfully.

Author: roomote