feat:(pdfa-conversion) Implement Strict PDF/A Mode with Verification (#5663)

# Description of Changes


This PR introduces a new "Strict Mode" for the PDF to PDF/A conversion
tool. When enabled, the application will use VeraPDF to verify that the
resulting file is perfectly compliant with the selected PDF/A standard.
If validation fails, the system will return a descriptive error instead
of a non-compliant file.



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---

## Checklist

### General

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/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/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [ ] I have performed a self-review of my own code
- [ ] 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/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### Translations (if applicable)

- [ ] I ran
[`scripts/counter_translation.py`](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/docs/counter_translation.md)

### 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/devGuide/DeveloperGuide.md#6-testing)
for more details.

---------

Signed-off-by: Balázs Szücs <bszucs1209@gmail.com>

frontend/src/core/tests/convert/ConvertSmartDetectionIntegration.test.tsx

@@ -396,7 +396,8 @@ describe('Convert Tool - Smart Detection Integration Tests', () => {
         paramsResult.current.updateParameter('fromExtension', 'pdf');
         paramsResult.current.updateParameter('toExtension', 'pdfa');
         paramsResult.current.updateParameter('pdfaOptions', {
-          outputFormat: 'pdfa'
+          outputFormat: 'pdfa',
+          strict: false
         });
       });
 
@@ -409,6 +410,7 @@ describe('Convert Tool - Smart Detection Integration Tests', () => {
 
       const formData = (mockedApiClient.post as Mock).mock.calls[0][1] as FormData;
       expect(formData.get('outputFormat')).toBe('pdfa');
+      expect(formData.get('strict')).toBe('false');
       expect(mockedApiClient.post).toHaveBeenCalledWith('/api/v1/convert/pdf/pdfa', expect.any(FormData), {
         responseType: 'blob'
       });