fix: Subscription diagnostic nodes not deleted when disabling diagnostics

[RicoSuter]

Fix a bug in DiagnosticsNodeManager.SetDiagnosticsEnabled(false) where the subscription cleanup loop indexes m_sessions instead of m_subscriptions.

Disclaimer: This bug has been found with hours of manual work with AI and my local OPC UA stress test application and should be safe - please review the changes as I don't really know the OPC UA code base in depth.

Bug

When SetDiagnosticsEnabled(false) is called, the subscription cleanup loop at line 643-646 iterates m_subscriptions.Count but indexes m_sessions[ii]:

for (int ii = 0; ii < m_subscriptions.Count; ii++)
{
    nodesToDelete.Add(m_sessions[ii].Value.Variable); // should be m_subscriptions[ii]
}

This causes:

• IndexOutOfRangeException when there are more subscriptions than sessions, preventing any cleanup
• Wrong nodes deleted otherwise — session nodes are added to the delete list instead of subscription nodes

Subscription diagnostic nodes (and their child states and references) remain permanently in PredefinedNodes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64.72%. Comparing base (d25caff) to head (c10cc07).
⚠️ Report is 118 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #3559       +/-   ##
===========================================
+ Coverage   51.86%   64.72%   +12.85%     
===========================================
  Files         370      458       +88     
  Lines       78618    96773    +18155     
  Branches    13650    16264     +2614     
===========================================
+ Hits        40779    62637    +21858     
+ Misses      33705    29138     -4567     
- Partials     4134     4998      +864     

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