fix(FN-920): improve error handling in subtask breakdown SSE stream

- Handle empty error messages in subtask-breakdown session by falling back to 'Unknown error'
- Fix backend SSE routes to coerce error data to string and handle missing message
- Improve frontend connectSubtaskStream to validate parsed error data before displaying
- Add tests for error display in SubtaskBreakdownModal component

packages/dashboard/src/routes.ts

@@ -4950,7 +4950,7 @@ export function createApiRoutes(store: TaskStore, options?: ServerOptions): Rout
       }
 
       if (session.status === "error") {
-        res.write(`event: error\ndata: ${JSON.stringify(session.error || "Failed to generate subtasks")}\n\n`);
+        res.write(`event: error\ndata: ${JSON.stringify(String(session.error || "Unknown error"))}\n\n`);
         unsubscribe();
         res.end();
         return;
@@ -4969,7 +4969,7 @@ export function createApiRoutes(store: TaskStore, options?: ServerOptions): Rout
         unsubscribe();
       });
     } catch (err: any) {
-      res.write(`event: error\ndata: ${JSON.stringify(err.message || "Stream error")}\n\n`);
+      res.write(`event: error\ndata: ${JSON.stringify(String(err?.message) || "Unknown error")}\n\n`);
       res.end();
     }
   });

packages/dashboard/src/subtask-breakdown.ts

@@ -195,7 +195,7 @@ export async function createSubtaskSession(initialDescription: string, _store?:
     const existing = sessions.get(sessionId);
     if (!existing) return;
     existing.status = "error";
-    existing.error = err instanceof Error ? err.message : "Failed to generate subtasks";
+    existing.error = err instanceof Error ? (err.message || "Unknown error") : "Failed to generate subtasks";
     existing.updatedAt = new Date();
     persistSubtaskSession(existing, "error", existing.error);
     subtaskStreamManager.broadcast(sessionId, { type: "error", data: existing.error });