…odels, 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.

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## 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.