test(core): add comprehensive unit tests for controllers, services, models, and utilities (#4160)

# Description of Changes

- **What was changed**
- **CI**: Enhanced `build.yml` to publish JaCoCo coverage and post a PR
summary comment per matrix job (Spring Security/JDK). Also archives
JaCoCo XML reports alongside existing test results.
- **Tests (new & expanded)**: Added a broad set of unit tests across
`app/common`, `app/core`, and `app/proprietary` modules, e.g.:
- Common: `ShowAdminInterfaceTest`, `UnsupportedClaimExceptionTest`,
`ExceptionUtilsTest`, `TempDirectoryTest`, etc.
- Core: `ConnectedInputStreamTest`, `ReplaceAndInvertColorFactoryTest`,
controller/model/service tests (e.g. `SettingsControllerTest`,
`ApiEndpointTest`, `FlexibleCSVWriterTest`,
`MetricsAggregatorServiceTest`, etc.).
- Proprietary: security/database/model/web tests (e.g.
`H2SQLConditionTest`, `JPATokenRepositoryImplTest`,
`AuditWebFilterTest`, `CorrelationIdFilterTest`, etc.).
- **JUnit 5 cleanup**: Consolidated assertion imports (`import static
org.junit.jupiter.api.Assertions.*`), standardized on Jupiter APIs, and
minor Mockito/Jupiter setup tweaks.
- **Fix**: `ReplaceAndInvertColorFactory` now safely returns `null` when
`replaceAndInvertOption` is `null` to avoid NPEs.
- **Testability refactor**: Broadened visibility of
`SPDFApplication#getActiveProfile(String[] args)` (from `private` to
`protected`) to enable direct unit testing.
  - **Chore**: Removed obsolete `ValidationUtil` from `app/common`.

- **Why the change was made**
  - Improve **signal in PRs** via automatic coverage summaries.
- Increase **test coverage** and reduce regressions across core and
proprietary modules.
- Eliminate a potential **NullPointerException** in color strategy
selection.
- Enable targeted testing of application startup/profile resolution
logic.

---

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

.github/workflows/build.yml

@@ -110,19 +110,33 @@ jobs:
         with:
           name: test-reports-jdk-${{ matrix.jdk-version }}-spring-security-${{ matrix.spring-security }}
           path: |
+            app/core/build/reports/jacoco/test
             app/core/build/reports/tests/
             app/core/build/test-results/
             app/core/build/reports/problems/
             app/common/build/reports/tests/
             app/common/build/test-results/
+            app/common/build/reports/jacoco/test
             app/common/build/reports/problems/
             app/proprietary/build/reports/tests/
             app/proprietary/build/test-results/
+            app/proprietary/build/reports/jacoco/test
             app/proprietary/build/reports/problems/
             build/reports/problems/
           retention-days: 3
           if-no-files-found: warn
 
+      - name: Add coverage to PR with spring security ${{ matrix.spring-security }} and JDK ${{ matrix.jdk-version }}
+        id: jacoco
+        uses: madrapps/jacoco-report@50d3aff4548aa991e6753342d9ba291084e63848 # v1.7.2
+        with:
+          paths: |
+            ${{ github.workspace }}/**/build/reports/jacoco/test/jacocoTestReport.xml
+          token: ${{ secrets.GITHUB_TOKEN }}
+          min-coverage-overall: 10
+          min-coverage-changed-files: 0
+          comment-type: summary
+
   check-generateOpenApiDocs:
     if: needs.files-changed.outputs.openapi == 'true'
     needs: [files-changed, build]