fix(gateway): discard empty placeholder when voice transcription succeeds

When a Discord voice message arrives, the adapter sets event.text to
"(The user sent a message with no text content)" since voice messages
have no text content. The transcription enrichment in
_enrich_message_with_transcription() then prepends the transcript but
leaves the placeholder intact, causing the agent to receive both:

    [The user sent a voice message~ Here's what they said: "..."]

    (The user sent a message with no text content)

The agent sees this as two separate user turns — one transcribed
and one empty — creating confusing duplicate messages.

Fix: when the transcription succeeds and user_text is only the empty
placeholder, return just the transcript without the redundant placeholder.

gateway/run.py

@@ -6044,6 +6044,11 @@ class GatewayRunner:
 
         if enriched_parts:
             prefix = "\n\n".join(enriched_parts)
+            # Strip the empty-content placeholder from the Discord adapter
+            # when we successfully transcribed the audio — it's redundant.
+            _placeholder = "(The user sent a message with no text content)"
+            if user_text and user_text.strip() == _placeholder:
+                return prefix
             if user_text:
                 return f"{prefix}\n\n{user_text}"
             return prefix