RFR: 8351000: StringBuilder getChar and putChar robustness [v2]

[Chen Liang]

On Wed, 30 Apr 2025 13:13:30 GMT, Roger Riggs <rriggs at openjdk.org> wrote:

>> Improve StringBuilder getChar and putChar robustness with racy updates to StringBuilder. 
>> Test racing threads with append, insert, and reading characters.
>
> Roger Riggs has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Test cleanup

Good cleanup, thanks @turbanoff!

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Marked as reviewed by liach (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/24951#pullrequestreview-2807026031