fix(zulip): remove invalid kwargs from SendResult calls

SendResult only accepts: success, message_id, error, raw_response,
retryable. The adapter was passing platform=, chat_id=, and metadata=
which are not valid fields, causing TypeError on every send attempt.

This was the error Tanko was showing in Zulip:
'SendResult.__init__() got an unexpected keyword argument "platform"'

plugins/platforms/zulip/adapter.py

@@ -765,10 +765,7 @@ class ZulipAdapter(BasePlatformAdapter):
                 self._health_stats["send_count"] += 1
                 return SendResult(
                     success=True,
-                    platform="zulip",
-                    chat_id=chat_id,
                     message_id=msg_id,
-                    metadata={"placeholder": True},
                 )
 
         # Send actual content
@@ -777,16 +774,12 @@ class ZulipAdapter(BasePlatformAdapter):
             self._health_stats["send_count"] += 1
             return SendResult(
                 success=True,
-                platform="zulip",
-                chat_id=chat_id,
                 message_id=str(result["id"]),
             )
 
         self._health_stats["send_errors"] += 1
         return SendResult(
             success=False,
-            platform="zulip",
-            chat_id=chat_id,
             error="Failed to send message",
         )