fix: clean up transaction on send failure in _executeTransaction

Move _addMessage inside the try-catch block so that if writing to a
closed socket throws WebtritSignalingBadStateException, the transaction
is properly removed from the _transactions map instead of leaking.

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

Author: SERDUN

packages/webtrit_signaling/lib/src/webtrit_signaling_client.dart

@@ -248,9 +248,9 @@ class WebtritSignalingClient {
       requestJson['transaction'] = transaction.id;
     }
 
-    _addMessage(requestJson);
-
     try {
+      _addMessage(requestJson);
+
       final responseJson = await transaction.future;
       if (transaction.isIdGenerate) {
         responseJson.remove('transaction');