ZOOKEEPER-4947: Stop the send loop after SASL authentication fails - #2443

After SASL authentication fails, the send thread can remain in a transport wait. It eventually exits; the event thread already stops. Break out at the authentication-failure branch before another transport wait, so the existing cleanup runs without caller close. The client retains AUTH_FAILED; explicit close still sets CLOSED.

https://issues.apache.org/jira/browse/ZOOKEEPER-4947

Testing Done

The bad-DIGEST fixture uses a real file-backed loopback server and a 30-second session. It logs shutdown before close with a one-second bound per thread, then asserts shutdown, the retained failure state and request error.

From this PR's checkout, apply only its test changes to the baseline:

base=5a201d633fd90a14ba76795683be36b05baedff0
git worktree add --detach ../zk-auth-before "$base"
git diff "$base" HEAD -- zookeeper-server/src/test/java | git -C ../zk-auth-before apply

With JDK 25 and cached Maven dependencies, run this in the baseline and PR checkouts:

for socket in NIO Netty; do
  if mvn -B -ntp --offline -T 1 -Pfull-build -pl zookeeper-server -am test \
    -Dtest=SaslAuthFailTest#testBadSaslAuthNotifiesWatch \
    "-Dzookeeper.clientCnxnSocket=org.apache.zookeeper.ClientCnxnSocket${socket}" \
    -Dsurefire-forkcount=1 -Dsurefire.rerunFailingTestsCount=0 \
    -Dsurefire.failIfNoSpecifiedTests=false -DfailIfNoTests=false \
    -DredirectTestOutputToFile=false; then status=0; else status=$?; fi
  echo "$socket exit=$status"
done

Both baseline runs exited 1 with:

org.opentest4j.AssertionFailedError: Client threads should stop after SASL authentication fails ==> expected: <true> but was: <false>

The baseline observations below are NIO followed by Netty:

2026-09-19 23:59:20,939 [myid:] - INFO  [main:o.a.z.t.SaslAuthFailTest@101] - SASL failure shutdown without close: threadsStopped=false, state=AUTH_FAILED
2026-09-19 23:59:30,905 [myid:] - INFO  [main:o.a.z.t.SaslAuthFailTest@101] - SASL failure shutdown without close: threadsStopped=false, state=AUTH_FAILED

After the fix, both runs exited 0:

2026-09-19 23:59:39,562 [myid:] - INFO  [main:o.a.z.t.SaslAuthFailTest@101] - SASL failure shutdown without close: threadsStopped=true, state=AUTH_FAILED
2026-09-19 23:59:48,097 [myid:] - INFO  [main:o.a.z.t.SaslAuthFailTest@101] - SASL failure shutdown without close: threadsStopped=true, state=AUTH_FAILED