JUnits JUnits JUnits, so many JUnits (#3537)

# Description of Changes

Please provide a summary of the changes, including:

- What was changed
- Why the change was made
- Any challenges encountered

Closes #(issue_number)

---

## 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/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)
- [ ] 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/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.

---------

Co-authored-by: Copilot <[email protected]>
Co-authored-by: pixeebot[bot] <104101892+pixeebot[bot]@users.noreply.github.com>

build.gradle

@@ -1,5 +1,6 @@
 plugins {
     id "java"
+    id 'jacoco'
     id "org.springframework.boot" version "3.4.5"
     id "io.spring.dependency-management" version "1.1.7"
     id "org.springdoc.openapi-gradle-plugin" version "1.9.0"
@@ -542,6 +543,10 @@ dependencies {
     compileOnly "org.projectlombok:lombok:$lombokVersion"
     annotationProcessor "org.projectlombok:lombok:$lombokVersion"
 
+  // Mockito (core)
+    testImplementation 'org.mockito:mockito-core:5.11.0'
+
+    
     testRuntimeOnly 'org.mockito:mockito-inline:5.2.0'
 }