fix: handle edge case where JSON.stringify returns undefined

Add fallback to String() when JSON.stringify returns undefined,
which can happen with objects that have toJSON() returning undefined.

packages/cli/src/utils/errors.test.ts

@@ -125,6 +125,15 @@ describe('errors', () => {
       const obj = { message: 123 };
       expect(getErrorMessage(obj)).toBe('{"message":123}');
     });
+
+    it('should fallback to String() when toJSON returns undefined', () => {
+      const obj = {
+        toJSON() {
+          return undefined;
+        },
+      };
+      expect(getErrorMessage(obj)).toBe('[object Object]');
+    });
   });
 
   describe('handleError', () => {

packages/cli/src/utils/errors.ts

@@ -31,7 +31,9 @@ export function getErrorMessage(error: unknown): string {
   // Handle plain objects by stringifying them
   if (error !== null && typeof error === 'object') {
     try {
-      return JSON.stringify(error);
+      const stringified = JSON.stringify(error);
+      // JSON.stringify can return undefined for objects with toJSON() returning undefined
+      return stringified ?? String(error);
     } catch {
       // If JSON.stringify fails (circular reference, etc.), fall back to String
       return String(error);