fix(heartbeat): keep native chat compatibility fallback closed

Co-Authored-By: Paperclip <noreply@paperclip.ing>

server/src/__tests__/heartbeat-run-summary.test.ts

@@ -408,6 +408,9 @@ describe("resolveHeartbeatRunResponse", () => {
       expect(resolveHeartbeatRunResponse({ resultJson, nativeConversationFinalResponse: true,
         existingComment: { id: "explicit", body: "The release is ready at /releases/latest." },
       })).toMatchObject({ text: "The release is ready at /releases/latest.", decision: { commentAction: "reuse" } });
+      expect(resolveHeartbeatRunResponse({ resultJson, nativeConversationFinalResponse: true,
+        finalAgentMessage: { text: "Unknown narration", sourceEventId: "unknown", channel: "unknown" },
+      })).toMatchObject({ text: null, decision: { commentAction: "none" } });
     }
   });
 

server/src/services/heartbeat-run-summary.ts

@@ -585,6 +585,10 @@ export function resolveHeartbeatRunResponse(input: {
         reasonCodes: [...final.decision.reasonCodes, "native_conversation_final_precedence"],
       } };
     }
+    if (!existingText) return {
+      text: null,
+      decision: decision("none", { commentAction: "none", reasonCodes: ["native_conversation_final_response_unavailable"] }),
+    };
   }
   if (input.existingComment && existingText) {
     return {