RFR: 8369595: HttpClient: HttpHeaders.firstValueAsLong failures should be converted to ProtocolException [v3]

[Volkan Yazici]

> Add guards against `HttpHeaders::firstValueAsLong` failures. `H3MalformedResponseTest` is overhauled – migrated to JUnit, reinforced with exception type tests, etc. – but not all `firstValueAsLong` changes are verified with new additional tests. `test/jdk/java/net/httpclient/` tests still do pass.

Volkan Yazici has updated the pull request incrementally with four additional commits since the last revision:

 - Remove redundant `finally`
 - Use `-1` as default in `Http3ExchangeImpl`
 - Use `readStatusCode()` in `Http3Stream`
 - Replace `orElse(null)` with `Optional` usage

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Changes:
  - all: https://git.openjdk.org/jdk/pull/28431/files
  - new: https://git.openjdk.org/jdk/pull/28431/files/b491c9bc..0c384717

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28431&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28431&range=01-02

  Stats: 63 lines in 6 files changed: 23 ins; 31 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/28431.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28431/head:pull/28431

PR: https://git.openjdk.org/jdk/pull/28431