RFR: 8369812: HttpClient doesn't handle H3_REQUEST_REJECTED correctly

[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.

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Commit messages:
 - 8369812: HttpClient doesn't handle H3_REQUEST_REJECTED correctly

Changes: https://git.openjdk.org/jdk/pull/27793/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27793&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8369812
  Stats: 222 lines in 3 files changed: 215 ins; 2 del; 5 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