Run tests on matrix JDK via Gradle toolchains while keeping daemon on JDK 21

[Copilot]

Each CI matrix entry was silently running tests on JDK 21 regardless of matrix.java, because org.gradle.java.home forced the Gradle daemon (and thus test execution) onto JDK 21. Tests now actually run on the matrix JDK using Test.javaLauncher.

Changes

• build.gradle: Configures javaLauncher on the test task via Gradle toolchains when the testJavaVersion property is set, decoupling the test JVM from the Gradle daemon JVM:

  if (project.hasProperty('testJavaVersion')) {
      javaLauncher = javaToolchains.launcherFor {
          languageVersion = JavaLanguageVersion.of(project.property('testJavaVersion') as Integer)
      }
  }

• .github/workflows/gradle.yml: Passes -PtestJavaVersion=${{ matrix.java }} to both ./gradlew build invocations so each matrix entry (11, 17, 21, 25) exercises the corresponding JDK runtime. Gradle daemon remains on JDK 21 via org.gradle.java.home.

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.