test: account for CRT sync 5xx connection rotation

[dkropachev]

Summary

• document the AWS CRT sync 5xx connection reuse behavior in README
• allow the CRT sync client to rotate one connection after HTTP 5xx responses
• keep the test asserting that 5xx responses do not force one connection per request in the current probe

CRT 5xx behavior checked

• AWS SDK CRT sync response handling treats HTTP 5xx server errors as a condition where the native CRT connection may be closed instead of returned to the pool.
• There is no public AwsCrtHttpClient.Builder option to disable that status-code close behavior.
• Local Java 11 runs with AWS SDK 2.42.2 observed 1 TCP connection for five sequential 500 responses across five repeated runs.
• CI previously observed 2 TCP connections for the same five-response probe, so the assertion permits one replacement connection while still catching per-request connection churn.

Testing

• JAVA_HOME=/usr/lib/jvm/java-11-openjdk-amd64 PATH=/usr/lib/jvm/java-11-openjdk-amd64/bin:$PATH mvn -q -Dtest=HttpClientNon2xxConnectionReuseTest test
• JAVA_HOME=/usr/lib/jvm/java-11-openjdk-amd64 PATH=/usr/lib/jvm/java-11-openjdk-amd64/bin:$PATH mvn -q fmt:check checkstyle:check