RFR: 8372957: After JDK-8282441 JDWP might allow some invalid FrameIDs to be used

[Chris Plummer]

[JDK-8282441](https://bugs.openjdk.org/browse/JDK-8282441) added freeing of ThreadNodes for some vthreads that are still running. They are only suppose to be freed if the ThreadNode contains no state information that needs to be maintained. There appears to be a bug in the current logic, allowing the loss of the ThreadNode frameGeneration value when it should be retained. Details in first comment.

Tested with by running all tier5 CI svc tests, which includes virtual thread testing.

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Commit messages:
 - Don't free ThreadNodes that have generated FrameIDs.

Changes: https://git.openjdk.org/jdk/pull/28616/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28616&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372957
  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/28616.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28616/head:pull/28616

PR: https://git.openjdk.org/jdk/pull/28616