Preserve stack trace for CancelledSubscriptionException (#6525)

Motivation:
When a `CancelledSubscriptionException` was raised, it was created without a stack trace for performance reasons. This made it difficult for developers to debug the root cause of a subscription cancellation.

Modifications:
- Used `CancelledSubscriptionException.get()` when creating a `CloseEvent`.
  - This method leverages the configured `verboseExceptionSampler` to determine whether a full stack trace should be captured and preserved.

Result:
- Developers can now obtain a full stack trace for `CancelledSubscriptionException` by configuring the `verboseExceptionSampler`.

Author: minwoox

core/src/main/java/com/linecorp/armeria/common/stream/DefaultStreamMessage.java

@@ -290,10 +290,10 @@ private void doRequest(long n) {
     final void cancel() {
         if (setState(State.OPEN, State.CLEANUP) || setState(State.CLOSED, State.CLEANUP)) {
             // It the state was CLOSED, close() or close(cause) has been called before cancel() or abort()
-            // is called. We just ignore the previously pushed event and deal with CANCELLED_CLOSE.
+            // is called. We just ignore the previously pushed event and deal with cancelled close event.
             final SubscriptionImpl subscription = this.subscription;
             assert subscription != null;
-            notifySubscriberOfCloseEvent(subscription, CANCELLED_CLOSE);
+            notifySubscriberOfCloseEvent(subscription, newCloseEvent(CancelledSubscriptionException.get()));
         }
     }