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/test/java/stirling/software/SPDF/utils/FileToPdfTest.java

@@ -5,31 +5,79 @@ import stirling.software.SPDF.model.api.converters.HTMLToPdfRequest;
 
 import java.io.IOException;
 
-import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.*;
 
 public class FileToPdfTest {
 
+    /**
+     * Test the HTML to PDF conversion.
+     * This test expects an IOException when an empty HTML input is provided.
+     */
     @Test
     public void testConvertHtmlToPdf() {
         HTMLToPdfRequest request = new HTMLToPdfRequest();
-        byte[] fileBytes = new byte[0]; // Sample file bytes
-        String fileName = "test.html"; // Sample file name
-        boolean disableSanitize = false; // Sample boolean value
+        byte[] fileBytes = new byte[0]; // Sample file bytes (empty input)
+        String fileName = "test.html"; // Sample file name indicating an HTML file
+        boolean disableSanitize = false; // Flag to control sanitization
 
-        // Check if the method throws IOException
-        assertThrows(IOException.class, () -> {
-            FileToPdf.convertHtmlToPdf("/path/",request, fileBytes, fileName, disableSanitize);
-        });
+        // Expect an IOException to be thrown due to empty input
+        Throwable thrown =
+                assertThrows(
+                        IOException.class,
+                        () ->
+                                FileToPdf.convertHtmlToPdf(
+                                        "/path/", request, fileBytes, fileName, disableSanitize));
+        assertNotNull(thrown);
     }
 
+    /**
+     * Test sanitizeZipFilename with null or empty input.
+     * It should return an empty string in these cases.
+     */
     @Test
-    public void testConvertBookTypeToPdf() {
-        byte[] bytes = new byte[10]; // Sample bytes
-        String originalFilename = "test.epub"; // Sample original filename
+    public void testSanitizeZipFilename_NullOrEmpty() {
+        assertEquals("", FileToPdf.sanitizeZipFilename(null));
+        assertEquals("", FileToPdf.sanitizeZipFilename("   "));
+    }
 
-        // Check if the method throws IOException
-        assertThrows(IOException.class, () -> {
-            FileToPdf.convertBookTypeToPdf(bytes, originalFilename);
-        });
+    /**
+     * Test sanitizeZipFilename to ensure it removes path traversal sequences.
+     * This includes removing both forward and backward slash sequences.
+     */
+    @Test
+    public void testSanitizeZipFilename_RemovesTraversalSequences() {
+        String input = "../some/../path/..\\to\\file.txt";
+        String expected = "some/path/to/file.txt";
+
+        // Print output for debugging purposes
+        System.out.println("sanitizeZipFilename " + FileToPdf.sanitizeZipFilename(input));
+        System.out.flush();
+
+        // Expect that the method replaces backslashes with forward slashes
+        // and removes path traversal sequences
+        assertEquals(expected, FileToPdf.sanitizeZipFilename(input));
+    }
+
+    /**
+     * Test sanitizeZipFilename to ensure that it removes leading drive letters and slashes.
+     */
+    @Test
+    public void testSanitizeZipFilename_RemovesLeadingDriveAndSlashes() {
+        String input = "C:\\folder\\file.txt";
+        String expected = "folder/file.txt";
+        assertEquals(expected, FileToPdf.sanitizeZipFilename(input));
+
+        input = "/folder/file.txt";
+        expected = "folder/file.txt";
+        assertEquals(expected, FileToPdf.sanitizeZipFilename(input));
+    }
+
+    /**
+     * Test sanitizeZipFilename to verify that safe filenames remain unchanged.
+     */
+    @Test
+    public void testSanitizeZipFilename_NoChangeForSafeNames() {
+        String input = "folder/subfolder/file.txt";
+        assertEquals(input, FileToPdf.sanitizeZipFilename(input));
     }
 }