fix: fallback agent payload source to numeric value

Keep Lingma chat/ask payload source as numeric 1 for agent mode A/B validation against remote upstream timeout behavior.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

app/lingma_client.py

@@ -904,7 +904,7 @@ class LingmaGatewayClient:
             "chatTask": "chat" if ask_mode == "agent" else "FREE_INPUT",
             "mode": ask_mode,
             "stream": True,
-            "source": "vscode" if ask_mode == "agent" else 1,
+            "source": 1,
             "isReply": is_reply,
             "taskDefinitionType": "system",
             "content": prompt,