fix(saml): correct ClassPathResource handling for IdP metadata and add null-guard for privateKey (#4157)

## Description of Changes

**What was changed**
- In `getIdpMetadataUri()`, use
`idpMetadataUri.substring("classpath:".length())` so the `classpath:`
scheme (including the colon) is stripped correctly before creating the
`ClassPathResource`.
- In `getPrivateKey()`, add a null check (`if (privateKey == null)
return null;`) to avoid a potential `NullPointerException` when the
property is unset.

**Why the change was made**
- The previous substring used `"classpath".length()` (without the
colon), leaving a leading `:` in the path (e.g., `:/saml/idp.xml`) which
breaks `ClassPathResource` resolution and can prevent SAML bootstrapping
when `idpMetadataUri` uses the `classpath:` scheme.
- The null-guard aligns the method with defensive coding practices and
prevents runtime errors when no private key is configured.


---

## 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.

build.gradle

@@ -69,7 +69,7 @@ allprojects {
 tasks.register('writeVersion', WriteProperties) {
     outputFile = layout.projectDirectory.file('app/common/src/main/resources/version.properties')
     println "Writing version.properties to ${outputFile.path}"
-    comment "${new Date()}"
+    comment = "${new Date()}"
     property 'version', project.provider { project.version.toString() }
 }
 
@@ -128,6 +128,9 @@ subprojects {
         testImplementation 'org.springframework.boot:spring-boot-starter-test'
         testRuntimeOnly 'org.mockito:mockito-inline:5.2.0'
         testRuntimeOnly "org.junit.platform:junit-platform-launcher:$junitPlatformVersion"
+
+        testImplementation platform("com.squareup.okhttp3:okhttp-bom:5.1.0")
+        testImplementation "com.squareup.okhttp3:mockwebserver"
     }
 
     tasks.withType(JavaCompile).configureEach {
@@ -153,6 +156,17 @@ subprojects {
         }
     }
 
+    jacocoTestCoverageVerification {
+        dependsOn jacocoTestReport
+        violationRules {
+            rule {
+                limit {
+                    minimum = 0.0
+                }
+            }
+        }
+    }
+
     tasks.named("processResources") {
         dependsOn(rootProject.tasks.writeVersion)
     }
@@ -569,6 +583,9 @@ dependencies {
 
     testImplementation 'org.springframework.boot:spring-boot-starter-test'
     testRuntimeOnly "org.junit.platform:junit-platform-launcher:$junitPlatformVersion"
+
+    testImplementation platform("com.squareup.okhttp3:okhttp-bom:5.1.0")
+    testImplementation "com.squareup.okhttp3:mockwebserver"
 }
 
 tasks.named("test") {