fix: Clear delivery candidates on cancel

This change ensures that when the `ChannelDeliveryReporter` is canceled, any pending delivery candidates are cleared.

The `cancel` method now clears the `_deliveryCandidates` list in addition to canceling the throttled reporting function. This prevents a potential memory leak and ensures a clean state when delivery reporting is stopped.

Author: xsahil03x

packages/stream_chat/lib/src/client/channel_delivery_reporter.dart

@@ -193,5 +193,8 @@ class ChannelDeliveryReporter {
   ///
   /// Typically used when shutting down the reporter or permanently stopping
   /// delivery reporting.
-  void cancel() => _throttledMarkCandidatesAsDelivered.cancel();
+  void cancel() {
+    _throttledMarkCandidatesAsDelivered.cancel();
+    _deliveryCandidatesLock.synchronized(_deliveryCandidates.clear);
+  }
 }