Improve Type Safety and OpenAPI Schema for PDF API Controllers and Models (#3470)

# Description of Changes

- **What was changed**  
- Updated controller methods to use strongly‐typed primitives (`int`,
`long`, `boolean`) instead of `String` for numeric and boolean
parameters, eliminating calls to `Integer.parseInt`/`Long.parseLong` and
improving null‐safety (`Boolean.TRUE.equals(...)`).
- Enhanced all API request model classes with richer Swagger/OpenAPI
annotations: added `requiredMode`, `defaultValue`, `allowableValues`,
`format`, `pattern`, and tightened schema descriptions for all fields.
- Refactored HTML form templates for “Remove Blank Pages” to include
`min`, `max`, and `step` attributes on numeric inputs, matching the
updated validation rules.

- **Why the change was made**  
- **Type safety & robustness**: Shifting from `String` to native types
prevents runtime parsing errors, simplifies controller logic, and makes
default values explicit.
- **Better API documentation & validation**: Enriching the Swagger
annotations ensures generated docs accurately reflect required fields,
default values, and permitted ranges, which improves client code
generation and developer experience.
- **Consistency across codebase**: Aligning all request models and
controllers enforces a uniform coding style and reduces bugs.

#3406

---

## 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/controller/api/MergeController.java

@@ -117,20 +117,20 @@ public class MergeController {
                     "This endpoint merges multiple PDF files into a single PDF file. The merged"
                             + " file will contain all pages from the input files in the order they were"
                             + " provided. Input:PDF Output:PDF Type:MISO")
-    public ResponseEntity<byte[]> mergePdfs(@ModelAttribute MergePdfsRequest form)
+    public ResponseEntity<byte[]> mergePdfs(@ModelAttribute MergePdfsRequest request)
             throws IOException {
         List<File> filesToDelete = new ArrayList<>(); // List of temporary files to delete
         File mergedTempFile = null;
         PDDocument mergedDocument = null;
 
-        boolean removeCertSign = form.isRemoveCertSign();
+        boolean removeCertSign = Boolean.TRUE.equals(request.getRemoveCertSign());
 
         try {
-            MultipartFile[] files = form.getFileInput();
+            MultipartFile[] files = request.getFileInput();
             Arrays.sort(
                     files,
                     getSortComparator(
-                            form.getSortType())); // Sort files based on the given sort type
+                            request.getSortType())); // Sort files based on the given sort type
 
             PDFMergerUtility mergerUtility = new PDFMergerUtility();
             long totalSize = 0;