RFR: 8358594: Misleading keyLength value captured in JFR event for ML-KEM key

[Weijun Wang]

Add more comment on why `KeyUtil::getKeySize` could return -1. Add a new method `getNistCategory` to get the NIST security category.

-------------

Commit messages:
 - the fix

Changes: https://git.openjdk.org/jdk/pull/25642/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25642&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8358594
  Stats: 112 lines in 2 files changed: 110 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/25642.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25642/head:pull/25642

PR: https://git.openjdk.org/jdk/pull/25642