fix: trigger onOpen callback in Swift WebSocket after upgrade completes

Previously, the onOpen callback was not being triggered when a WebSocket
connection was established. This was due to a lifecycle timing issue where
the MessageHandler's channelActive method was never called.

The issue occurred because:
1. HTTP channel becomes active first
2. HTTP-to-WebSocket upgrade happens
3. MessageHandler is added to the pipeline after the channel is already active
4. channelActive only fires when a handler is added to an inactive channel

The fix explicitly triggers the onOpen callback in the upgradePipelineHandler
method after successfully adding the MessageHandler to the pipeline, which is
exactly when the WebSocket connection is fully established.

This ensures the onOpen callback works correctly in all SDKs that depend on
this Swift WebSocket client, including the Apple (iOS/macOS) SDK.

Author: ChiragAgg5k

templates/swift/Sources/WebSockets/WebSocketClient.swift.twig

@@ -297,7 +297,13 @@ public class WebSocketClient {
             self.channel = channel
         }
 
-        return channel.pipeline.addHandler(handler)
+        return channel.pipeline.addHandler(handler).map {
+            if let delegate = self.delegate {
+                delegate.onOpen(channel: channel)
+            } else {
+                self.onOpen(channel)
+            }
+        }
     }
 
     // MARK: - Close connection