Problem

Under high concurrency, NioClient.processReadyKeys() could throw a ConcurrentModificationException when iterating over selector.selectedKeys(). This happens if another thread registers a new channel with the selector while the iteration is in progress.

Solution

• The method now copies selector.selectedKeys() to a new ArrayList before iterating.
• This prevents concurrent modification during iteration, as recommended by Java NIO best practices.
• The approach is safe, efficient, and matches the behavior of other high-performance NIO frameworks.

Test Coverage

• Already added regression test in Add concurrency test for processReadyKeys() to detect potential thread-safety issues dnsjava/dnsjava#380

Impact

• Prevents application crashes and instability under high load.
• No change in normal behavior; all ready keys are still processed as expected.

Closes: #