SonarQube S7504 flagged `for slot in list(self.engine.subscriptions)` in
ServerReader.shutdown as an unnecessary list() call. It is necessary:
engine.unsubscribe() pops from engine.subscriptions, so iterating the dict
directly raises "dictionary changed size during iteration" and leaves the
remaining slots subscribed. Kept the snapshot with an explanatory comment and
`# NOSONAR`, the same convention as OpcConnectionPool.close_all().
Adds a regression test where unsubscribe() really pops: it fails with
RuntimeError without the list() and passes with it.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
b2d106e5c4