Fix ObjectDisposedException in SubscriptionManager background tasks on shutdown

[Copilot]

Proposed changes

During server shutdown, background tasks (PublishSubscriptionsAsync and ConditionRefreshWorkerAsync) can race with Dispose(), attempting to access disposed SemaphoreSlim and ManualResetEvent objects. This causes ObjectDisposedException errors in logs despite being a benign shutdown sequence.

Solution: Catch ObjectDisposedException in both background tasks and log as normal shutdown instead of error.

catch (ObjectDisposedException)
{
    m_logger.LogInformation(
        "Subscription - Publish Task {TaskId:X8} Exited Normally (disposed during shutdown).",
        Task.CurrentId);
}

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• Enhancement (non-breaking change which adds functionality)
• Test enhancement (non-breaking change to increase test coverage)
• Breaking change (fix or feature that would cause existing functionality to not work as expected, requires version increase of Nuget packages)
• Documentation Update (if none of the other choices apply)

Checklist

• I have read the CONTRIBUTING doc.
• I have signed the CLA.
• I ran tests locally with my changes, all passed.
• I fixed all failing tests in the CI pipelines.
• I fixed all introduced issues with CodeQL and LGTM.
• I have added tests that prove my fix is effective or that my feature works and increased code coverage.
• I have added necessary documentation (if appropriate).
• Any dependent changes have been merged and published in downstream modules.

Further comments

No tests added - the race condition is timing-dependent and difficult to reproduce reliably. The fix is defensive: it handles an expected shutdown scenario gracefully rather than modifying coordination logic.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[Server] Fix ObjectDisposedException in SubscriptionPublishTask on shutdown</issue_title>
<issue_description>SubscriptionTest SubscriptionTest
SERVER 18:35:14.494 [Opc.Ua.Server.SubscriptionManager] Subscription - Publish Task (null) Exited Unexpectedly.
[ObjectDisposedException] Cannot access a disposed object.
Object name: 'System.Threading.SemaphoreSlim'.
--- at Opc.Ua.Server.SubscriptionManager.PublishSubscriptionsAsync(Int32 sleepCycle, CancellationToken cancellationToken) in /_/Libraries/Opc.Ua.Server/Subscription/SubscriptionManager.cs:line 2076
</issue_description>

<agent_instructions>Please fix the ObjectDisposedException in the SubscriptionManager</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes [Server] Fix ObjectDisposedException in SubscriptionPublishTask on shutdown #3455

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