Fix CI matrix: run tests on matrix JDK, not just the Gradle daemon JDK

[Copilot]

The CI matrix's Java 11/17/25 entries weren't actually running tests on those JDKs. The Gradle daemon was pinned to JDK 21 (required by settings.gradle's Java 17+ check), so compilation and test execution always happened on JDK 21. The "Launcher JVM: 11.0.30" in logs is just the thin Gradle client wrapper — the "Daemon JVM: JDK 21" is what runs the build.

Changes

• build.gradle: Read a testJavaExecutable Gradle property in the test task and use it to set the test JVM executable, allowing the daemon (JDK 21) and test JVM to differ:

  test {
    if (project.hasProperty("testJavaExecutable")) {
      executable = project.property("testJavaExecutable").toString()
    }
    // ...
  }

• .github/workflows/gradle.yml: Extend the "Use JDK 21 for Gradle" step to also write testJavaExecutable=$JAVA_HOME/bin/java into ~/.gradle/gradle.properties. At that point JAVA_HOME is the matrix JDK (last setup-java wins), so each matrix slot runs tests on its respective JDK while the Gradle daemon stays on JDK 21.

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