[TESTS] Add comprehensive tests for parse_update_entry function

## Summary
The `parse_update_entry` function in `src/util/mod.rs` parses update lines but could benefit from more comprehensive tests covering edge cases. Add tests for malformed input, package names with dashes, empty/whitespace input, and Unicode characters.

## Files to modify
- `src/util/mod.rs` (add test cases in the `#[cfg(test)]` module)

## Expected behavior
The test suite should cover:
1. **Valid inputs**: Standard pacman update format parsing
2. **Malformed inputs**: Missing arrows, malformed versions, incomplete entries
3. **Edge cases**: Package names with dashes, empty/whitespace input, Unicode characters
4. **Error handling**: Proper `None` returns for invalid formats

## Implementation approach
Add a new test function `util_parse_update_entry` with comprehensive test cases:

## Testing
- [ ] `cargo check` passes
- [ ] `cargo clippy --all-targets --all-features -- -D warnings` passes
- [ ] `cargo test -- --test-threads=1` passes
- [ ] Test valid parsing of standard pacman update format
- [ ] Test malformed inputs return `None`
- [ ] Test package names containing dashes are handled correctly
- [ ] Test empty and whitespace-only inputs return `None`
- [ ] Test Unicode characters in package names
- [ ] Test version numbers with various formats (numbers, letters, dashes)
- [ ] Test edge cases with extra whitespace around separators

## Additional context
The `parse_update_entry` function parses lines in the format:
```
"name - old_version -> name - new_version"
```

Key parsing logic:
- Uses `find(" -> ")` to locate the arrow separator
- Uses `rfind(" - ")` to find the last dash before the arrow (handles package names with dashes)
- Returns `None` for any malformed input

Test cases should cover real-world pacman output scenarios and ensure the function is robust against various input formats and edge cases that might occur in practice.