fix: bound fixed thread pool queue

[2769194950]

Description

Fixes #1283.

ThreadUtils.newFixedThreadPool delegated to Executors.newFixedThreadPool, which uses an effectively unbounded LinkedBlockingQueue. Under sustained ZooKeeper latency or callback bursts, submitted tasks could accumulate without a memory bound.

This changes the executor to a fixed-size ThreadPoolExecutor with:

• a bounded queue of 10,000 tasks;
• CallerRunsPolicy to apply backpressure without silently dropping work;
• the existing thread count and Curator thread factory.

Added a regression test that verifies the queue bound and rejection policy.

Testing

mvnw.cmd -ntp -pl curator-client -am -Dtest=TestThreadUtils -Dsurefire.failIfNoSpecifiedTests=false test

Result: TestThreadUtils 1/1 passed; the 3-module reactor completed successfully.