Fix ExtendedEnum ClassNotFoundException in ForkJoinPool (#2748)

[oranoran]

Replace RenameHandler.lookupType() with Class.forName() using explicit classloader in ExtendedEnum and CombinedExtendedEnum. RenameHandler relies on thread context classloader which is null in ForkJoinPool workers, causing ClassNotFoundException in production environments with isolated classloader hierarchies (application servers, OSGi).

The fix uses ExtendedEnum.class.getClassLoader() instead of TCCL, ensuring classes are loaded from the correct classloader regardless of threading context.

Testing:

• ExtendedEnumThreadSafetyTest: Verifies thread-safety patterns in various concurrent contexts (ForkJoinPool, parallel streams, CompletableFuture). This test is comprehensive and valuable, but it does not reproduce the bug, due to how classloading works under Surefire.
• ExtendedEnumClassLoaderIsolationTest: Reproduces the production bug by running in an isolated parent/child classloader hierarchy with null TCCL, executed automatically during Maven verify phase, outside of the regular Maven/Surefire context.

[oranoran]

I don't have access to see why the build has failed - can someone shed some light? Locally on my Mac, "mvn clean install" passes, including the new tests. I hope there is no issue in CircleCI with the exec-maven-plugin running a standalone Java process for the ExtendedEnum classloader isolation test. If there is - this can be dropped from the POM, losing the regression test as part of the CI build.