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.

testing/cucumber/features/examples.feature

@@ -2,7 +2,7 @@
 Feature: API Validation
 
   @positive @password
-  Scenario: Remove password 
+  Scenario: Remove password
     Given I generate a PDF file as "fileInput"
     And the pdf contains 3 pages
     And the pdf is encrypted with password "password123"
@@ -47,9 +47,9 @@ Feature: API Validation
     And the response file should have size greater than 100
     And the response PDF is passworded
 	And the response status code should be 200
-	
+
   @positive @password
-  Scenario: Add password with other params 
+  Scenario: Add password with other params
     Given I generate a PDF file as "fileInput"
     And the pdf contains 3 pages
     And the request data includes
@@ -64,7 +64,7 @@ Feature: API Validation
     And the response file should have size greater than 100
     And the response PDF is passworded
 	And the response status code should be 200
-	
+
   @positive @watermark
   Scenario: Add watermark
     Given I generate a PDF file as "fileInput"
@@ -78,6 +78,8 @@ Feature: API Validation
       | opacity       | 0.5              |
       | widthSpacer   | 50               |
       | heightSpacer  | 50               |
+      | alphabet      | roman            |
+      | customColor   | #d3d3d3        |
     When I send the API request to the endpoint "/api/v1/security/add-watermark"
     Then the response content type should be "application/pdf"
     And the response file should have size greater than 100
@@ -107,7 +109,7 @@ Feature: API Validation
     Then the response content type should be "application/pdf"
     And the response file should have size greater than 0
 	And the response status code should be 200
-	
+
   @positive @metadata
   Scenario: Update metadata
     Given I generate a PDF file as "fileInput"
@@ -127,4 +129,4 @@ Feature: API Validation
 	And the response PDF metadata should include "Title" as "Sample Title"
 	And the response status code should be 200
 
-  
+