[jdk21u-dev] RFR: 8318915: Enhance checks in BigDecimal.toPlainString()

[Goetz Lindenmaier]

I backport this for parity with 21.0.8-oracle

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Commit messages:
 - Backport a6785e4d633908596ddb6de6d2eeab1c9ebdf2c3

Changes: https://git.openjdk.org/jdk21u-dev/pull/1676/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=1676&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318915
  Stats: 63 lines in 2 files changed: 43 ins; 6 del; 14 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/1676.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/1676/head:pull/1676

PR: https://git.openjdk.org/jdk21u-dev/pull/1676