build(nui): declare junit-platform-launcher explicitly

Gradle 9 dropped the auto-supplied JUnit Platform Launcher test
dependency, so bumping the wrapper to 9.7.0 in this PR made
:nui:test fail immediately with 'Failed to load JUnit Platform' -
before any test even ran. Declare it explicitly as testRuntimeOnly,
version-paired with the existing junit-jupiter 5.6.2.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: soloturn

nui/build.gradle

@@ -32,6 +32,7 @@ dependencies {
     annotationProcessor "org.terasology.gestalt:gestalt-inject-java:8.0.0-SNAPSHOT"
 
     testRuntimeOnly("org.junit.jupiter:junit-jupiter-engine:5.6.2")
+    testRuntimeOnly("org.junit.platform:junit-platform-launcher:1.6.2")
     testImplementation("org.junit.jupiter:junit-jupiter-api:5.6.2")
     testImplementation("org.junit.jupiter:junit-jupiter-params:5.6.2")
     testImplementation("junit:junit:4.12")