fix(logging): flush buffered logs on close

[dengliming]

Summary

• stop accepting new log entries once collector shutdown begins
• drain default and per-selector queues in bounded final batches before closing the client
• add regression coverage for single-client ordering and all multi-client buffers

Testing

• mvn -q -pl shenyu-plugin/shenyu-plugin-logging/shenyu-plugin-logging-common -am -DskipTests=false -Dcheckstyle.skip=false -Dtest=AbstractLogCollectorTest -DfailIfNoTests=false test

Fixes #6763

[Aias00]

Approved as PMC (Aias00). Reviewed the diff — small, well-scoped fix with regression tests; mergeable.