fix(backend): yield result pin in MCP simulation error path

When simulate_mcp_block catches a RuntimeError/ValueError, it now yields
a ("result", None) before ("error", ...) so downstream nodes connected
to the result pin are not starved during dry-run error paths.

autogpt_platform/backend/backend/executor/simulator.py

@@ -373,6 +373,8 @@ async def simulate_mcp_block(
             if pin_value is not None and pin_value != "":
                 yield pin_name, pin_value
     except (RuntimeError, ValueError) as e:
+        # Yield both pins so downstream nodes connected to "result" aren't starved.
+        yield "result", None
         yield "error", str(e)