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.

---

## 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.

src/main/java/stirling/software/SPDF/utils/FileToPdf.java

@@ -169,7 +169,7 @@ public class FileToPdf {
             }
         }
 
-        // search for the main HTML file.
+        // Search for the main HTML file.
         try (Stream<Path> walk = Files.walk(tempDirectory)) {
             List<Path> htmlFiles =
                     walk.filter(file -> file.toString().endsWith(".html"))
@@ -190,46 +190,20 @@ public class FileToPdf {
         }
     }
 
-    public static byte[] convertBookTypeToPdf(byte[] bytes, String originalFilename)
-            throws IOException, InterruptedException {
-        if (originalFilename == null || originalFilename.lastIndexOf('.') == -1) {
-            throw new IllegalArgumentException("Invalid original filename.");
-        }
-
-        String fileExtension = originalFilename.substring(originalFilename.lastIndexOf('.'));
-        List<String> command = new ArrayList<>();
-        Path tempOutputFile = Files.createTempFile("output_", ".pdf");
-        Path tempInputFile = null;
-
-        try {
-            // Create temp file with appropriate extension
-            tempInputFile = Files.createTempFile("input_", fileExtension);
-            Files.write(tempInputFile, bytes);
-
-            command.add("ebook-convert");
-            command.add(tempInputFile.toString());
-            command.add(tempOutputFile.toString());
-            ProcessExecutorResult returnCode =
-                    ProcessExecutor.getInstance(ProcessExecutor.Processes.CALIBRE)
-                            .runCommandWithOutputHandling(command);
-
-            return Files.readAllBytes(tempOutputFile);
-        } finally {
-            // Clean up temporary files
-            if (tempInputFile != null) {
-                Files.deleteIfExists(tempInputFile);
-            }
-            Files.deleteIfExists(tempOutputFile);
-        }
-    }
-
     static String sanitizeZipFilename(String entryName) {
         if (entryName == null || entryName.trim().isEmpty()) {
-            return entryName;
+            return "";
         }
+        // Remove any drive letters (e.g., "C:\") and leading forward/backslashes
+        entryName = entryName.replaceAll("^[a-zA-Z]:[\\\\/]+", "");
+        entryName = entryName.replaceAll("^[\\\\/]+", "");
+
+        // Recursively remove path traversal sequences
         while (entryName.contains("../") || entryName.contains("..\\")) {
             entryName = entryName.replace("../", "").replace("..\\", "");
         }
+        // Normalize all backslashes to forward slashes
+        entryName = entryName.replaceAll("\\\\", "/");
         return entryName;
     }
 }