Fix native memory leak in RocksJava thread-local comparator buffers

[Shikha-code36]

Fixes a native memory leak in RocksJava: when a Comparator is configured with
thread-local reusable buffers (ReusedSynchronisationType.THREAD_LOCAL), the
native ThreadLocalBuf wrapper allocated per thread in
ComparatorJniCallback::GetBuffer was never freed.

The ThreadLocalPtr unref handler correctly released the JNI global reference
and the direct-buffer backing memory, but never deleted the ThreadLocalBuf
object itself:

UnrefHandler unref = [](void* ptr) {
  ThreadLocalBuf* tlb = reinterpret_cast<ThreadLocalBuf*>(ptr);
  ...
  _env->DeleteGlobalRef(tlb->jbuf);
  JniUtil::releaseJniEnv(tlb->jvm, attached_thread);
  // tlb itself was never deleted
};

Fixes #14975

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