SIENTIAPDE-1974: fix(read): mark the shutdown snapshot loop for SonarQube (S7504)

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>

ingestor/managers/server_reader.py

@@ -83,7 +83,9 @@ class ServerReader:
         also dropped, so the next initialization reconnects. Without it (the slot just
         moved to another pod) the connection stays for writes/gRPC: the connection is
         shared, and the idle sweep closes it if nobody uses it anymore."""
-        for slot in list(self.engine.subscriptions):
+        # list() snapshots the keys: unsubscribe() pops from engine.subscriptions, so
+        # iterating the dict directly would raise "changed size during iteration".
+        for slot in list(self.engine.subscriptions):  # NOSONAR
             await self.engine.unsubscribe(slot)
         if evict:
             await self.pool.evict(self.server_id)