RFR: 8369656 : Calling CompletableFuture.join() could execute task in common pool [v3]

[Viktor Klang]

> Addresses the regression regarding pool assistance and includes a regression test for `CompletableFuture::get()`, `CompletableFuture::get(timeout, unit)`, and `CompletableFuture::join()`

Viktor Klang has updated the pull request incrementally with one additional commit since the last revision:

  Changing name to help instead of assist; also fix some indentation.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/27800/files
  - new: https://git.openjdk.org/jdk/pull/27800/files/7fed8f87..55f09729

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27800&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27800&range=01-02

  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/27800.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27800/head:pull/27800

PR: https://git.openjdk.org/jdk/pull/27800