fix: suppress auto-instrumented AmqpReceiver TaskCanceledException APM errors during shutdown (5.7.5)

Root cause: Elastic APM creates "AzureServiceBus RECEIVE" transactions via DiagnosticSource
auto-instrumentation. The Azure SDK ends its Activity before firing ProcessErrorAsync, so
Agent.Tracer.CurrentTransaction is null in OnReceiveCancelled() — the TX ID is never tracked
and the filter passes the error through. Fix: add culprit-based suppression for
TaskCanceledException on the AmqpReceiver path (post-WebSockets fix, only fires on shutdown).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Robert Karp

docs/CHANGELOG.md

@@ -7,6 +7,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## 5.7.5
 - Fixed
   - `ApmTransactionManager` now registers the APM error filter **at construction time** (application startup) instead of lazily inside `OnReceiveCancelled()`. The lazy approach lost a race: during pod graceful shutdown the APM agent flushes its internal buffer concurrently with Service Bus processor teardown, so error events could be sent to APM before `ReceiverWrapper.OnExceptionOccured` ran and had a chance to register the filter. Registering at construction time — before any message processing starts — closes this window. A fallback call in `OnReceiveCancelled()` handles the edge case where the APM agent was not yet configured at construction.
+  - The filter now also suppresses `TaskCanceledException` / `OperationCanceledException` errors whose culprit originates in `AmqpReceiver.ReceiveMessagesAsyncInternal`. These error events are produced by Elastic APM's **auto-instrumented** Azure Service Bus transactions (`"AzureServiceBus RECEIVE from …"`): the Azure SDK ends its underlying `Activity` (and therefore the APM transaction) before calling `ProcessErrorAsync`, so `Agent.Tracer.CurrentTransaction` is already `null` when `OnReceiveCancelled()` runs — the transaction ID is never added to `_cancelledTransactionIds` and the transaction-ID-based filter path cannot suppress them. After switching to WebSockets transport, `TaskCanceledException` from this code path only occurs during pod graceful shutdown.
 
 ## 5.7.4
 - Fixed

src/Ev.ServiceBus.Apm/ApmTransactionManager.cs

@@ -120,9 +120,25 @@ private static void RegisterShutdownErrorFilter()
 
         // Returning null from the filter drops the error event before it reaches the APM server.
         Agent.AddFilter((IError error) =>
-            error.TransactionId is not null && _cancelledTransactionIds.ContainsKey(error.TransactionId)
-                ? null
-                : error);
+        {
+            // Case 1: transaction explicitly tracked via OnReceiveCancelled().
+            if (error.TransactionId is not null && _cancelledTransactionIds.ContainsKey(error.TransactionId))
+                return null;
+
+            // Case 2: Elastic APM auto-instrumented "AzureServiceBus RECEIVE" transactions.
+            // The Azure SDK ends its Activity (and therefore the APM transaction) before firing
+            // ProcessErrorAsync, so Agent.Tracer.CurrentTransaction is null by the time
+            // OnReceiveCancelled() runs — the transaction ID is never added to
+            // _cancelledTransactionIds. Identify these by culprit pattern instead.
+            // After switching to WebSockets transport, TaskCanceledException originating in
+            // AmqpReceiver.ReceiveMessagesAsyncInternal only occurs during pod graceful shutdown.
+            if (error.Exception?.Type is "System.Threading.Tasks.TaskCanceledException"
+                                       or "System.OperationCanceledException" &&
+                error.Culprit?.Contains("AmqpReceiver", StringComparison.Ordinal) == true)
+                return null;
+
+            return error;
+        });
     }
 
     private static bool IsTraceEnabled()