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

[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.

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Commit messages:
 - Improve reuse
 - Improve wording in test
 - Improve wording in test
 - Guard `firstValueAsLong()` usages
 - Assert thrown exception types
 - Convert to JUnit
 - Add QPACK crash course

Changes: https://git.openjdk.org/jdk/pull/28431/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28431&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8369595
  Stats: 653 lines in 7 files changed: 234 ins; 128 del; 291 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