fix(build): define repositories to resolve test classpath dependencies (#5650)

# Description of Changes

## What was changed
- Added an explicit `repositories { ... }` block in the root
`build.gradle` to ensure Gradle has repository definitions for resolving
dependencies.
- Included:
- Optional authenticated Maven repo (only when
`rootProject.ext.mavenUrl` is configured).
  - Shibboleth releases repository.
  - `mavenCentral()` as the default public repository.

## Why the change was made
- Fixes Gradle resolution failure:
- `Cannot resolve external dependency
org.springframework.boot:spring-boot-starter-test because no
repositories are defined.`
- Ensures `:testCompileClasspath` can resolve Spring Boot test
dependencies in all environments, including CI.

---

## 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.

build.gradle

@@ -461,6 +461,24 @@ swaggerhubUpload {
     oas = "3.0.0"  // The version of the OpenAPI Specification you"re using
 }
 
+repositories {
+    if (!rootProject.ext.mavenUrl.isEmpty()) {
+        maven {
+            url = rootProject.ext.mavenUrl + '/releases'
+            credentials(PasswordCredentials) {
+                username = rootProject.ext.username
+                password = rootProject.ext.password
+            }
+            authentication {
+                basic(BasicAuthentication)
+            }
+            allowInsecureProtocol = true
+        }
+    }
+    maven { url = "https://build.shibboleth.net/maven/releases" }
+    mavenCentral()
+}
+
 dependencies {
     implementation project(':stirling-pdf')
     implementation project(':common')