fix: Added KeyCount property in ListObjectsV2 action result, added a test case for one

backend/posix/posix.go

@@ -1317,6 +1317,7 @@ func (p *Posix) ListObjectsV2(input *s3.ListObjectsV2Input) (*s3.ListObjectsV2Ou
 		Name:                  &bucket,
 		NextContinuationToken: &results.NextMarker,
 		Prefix:                &prefix,
+		KeyCount:              int32(len(results.Objects)),
 	}, nil
 }