Remove `convertBookTypeToPdf` and Improve File Sanitization in `FileToPdf` (#3072)

# Description of Changes

Please provide a summary of the changes, including:

- **Removed `convertBookTypeToPdf` method**: 
- This method used `ebook-convert` from Calibre, which required external
dependencies.
- Its removal eliminates unnecessary process execution and simplifies
the codebase.
  
- **Enhanced `sanitizeZipFilename` function**:
  - Added handling for drive letters (e.g., `C:\`).
  - Ensured all slashes are normalized to forward slashes.
- Improved recursive path traversal removal to prevent directory escape
vulnerabilities.

- **Refactored `ProcessExecutor` output handling**:
  - Replaced redundant `.size() > 0` checks with `.isEmpty()`.
  
- **Expanded unit tests in `FileToPdfTest`**:
  - Added tests for `sanitizeZipFilename` to cover edge cases.
  - Improved test descriptions and added assertion messages.
  - Added debug print statements for easier test debugging.

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## Checklist

### General

- [x] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [x] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/HowToAddNewLanguage.md)
(if applicable)
- [x] I have performed a self-review of my own code
- [x] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md#6-testing)
for more details.