Target the published javac plugin at Java 8

[Copilot]

The plugin was built with a newer Java target and used post-Java-8 language features, which prevented it from working as a Java 8 javac plugin. This change keeps the Gradle build on a modern JDK while making the published plugin artifact Java 8-compatible.

• Build targeting

  • Compile plugin main sources with a Java 8 toolchain.
  • Resolve javac APIs from the Java 8 tools.jar so the plugin links against the Java 8 compiler surface.
  • Keep test/build execution on Java 17.

• Source compatibility

  • Replace pattern-matching instanceof usage in production code with Java 8-compatible casts and control flow.
  • Preserve existing constructor-invocation detection and stripping behavior.

• Compatibility guardrail

  • Add a test that asserts the emitted plugin classes target Java 8 bytecode (major version 52).

tasks.named<JavaCompile>("compileJava") {
    javaCompiler = java8Compiler
    sourceCompatibility = JavaVersion.VERSION_1_8.toString()
    targetCompatibility = JavaVersion.VERSION_1_8.toString()
}