RFR: 8369812: HttpClient doesn't handle H3_REQUEST_REJECTED correctly [v4]

[Jaikiran Pai]

> Can I please get a review of this change which proposes to fix the implementation in HttpClient to correctly handle the H3_REQUEST_REJECTED error code?
> 
> As noted in https://bugs.openjdk.org/browse/JDK-8369812, the current implementation in the HttpClient treats this like an error and causes the HTTP request to fail. The proposed change in this PR addresses this by marking a request which was reset due to H3_REQUEST_REJECTED as unprocessed. This then allows the `MultiExchange` to reissue the request afresh. This new behaviour matches the expectation specified for this error code in the HTTP/3 RFC.
> 
> A new jtreg test has been introduced to reproduce the issue and verify the fix.

Jaikiran Pai has updated the pull request incrementally with one additional commit since the last revision:

  Daniel's review - use POST instead of GET

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/27793/files
  - new: https://git.openjdk.org/jdk/pull/27793/files/80da00de..247fe0b0

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27793&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27793&range=02-03

  Stats: 6 lines in 1 file changed: 6 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/27793.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27793/head:pull/27793

PR: https://git.openjdk.org/jdk/pull/27793