[adityaanikam]

Fixes #3283

The transformers that TransformerManager registers returned the input classfileBuffer when no transformer had changed the class. For a retransform-capable agent, HotSpot treats a non-null return as a modification and keeps a copy of the original class bytes (JvmtiClassFileLoadHookPoster::post_to_env), so every loaded class kept an extra copy in native memory. Both the retransform-capable and the lazy transformer now return null when nothing changed.

Added TransformerManagerTest. The three cases that expect null fail without the change. I could not run the core module build on Windows because arthas-vmtool needs a native compile, so I ran the new test class against the same sources with JDK 21 and --release 8.

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