test(rules): add unit tests covering all public methods and edge cases

- Add comprehensive test coverage for all Rules public methods
- Test both success and error paths with proper assertions
- Add PartialEq derive to EolAction to enable comparisons in tests
- Include edge cases like duplicate labels and nonexistent IDs
- Mark file system integration tests as ignore to avoid interference

src/eol_action.rs

@@ -3,7 +3,7 @@ use std::fmt;
 /// End of life action
 /// - Trash - move the message to the trash to be automatically deleted by Google
 /// - Delete - delete the message immediately without allowing rescue from trash
-#[derive(Debug, Default, Clone)]
+#[derive(Debug, Default, Clone, PartialEq)]
 pub enum EolAction {
     #[default]
     /// Move the message to the trash