Fix unobserved task exceptions when pending messages overflow - #2272
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Server dispatch and session enqueue operations create packet bus items without waiting for delivery. When the pending-message queue overflows, Fail faults the item's completion task, which can later raise UnobservedTaskException even if the application handles the dropped-message notification. Observe the completion task's exception inside MqttPacketBusItem.Fail. This keeps the task faulted and preserves the original exception for existing and future WaitAsync callers, including DeliverApplicationMessageAsync. Add a garbage-collection regression test for failures without a waiter and verify that requesting the task before or after failure preserves the original overflow exception. Re-enable unobserved-exception tracking in the existing overflow integration tests for both drop strategies. Before the fix, the focused run failed the new unobserved-exception test and both enqueue overflow tests. After the fix, all nine focused cases and all 489 tests in the .NET 10 Release test suite pass. Fixes dotnet#2271
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When the server drops a queued message,
MqttPacketBusItem.Failfaults its completion task. Normal dispatch and session enqueue operations do not await that task, so the overflow can later reachTaskScheduler.UnobservedTaskExceptioneven when the application handles the dropped-message event.Observe the internal task's exception in
Failwhile leaving the task faulted. Existing and futureWaitAsynccallers still receive the original exception, including callers ofDeliverApplicationMessageAsync.Validation:
dotnet build MQTTnet.slnx --configuration Release: succeeds for .NET 8 and .NET 10, with 3 deprecation warnings in the unchanged benchmark project.Fixes #2271