Intermittent FAIL in `/cookies/partitioned-cookies/partitioned-cookies-samesite-attribute.https.html`

```
▶ OK /cookies/partitioned-cookies/partitioned-cookies-samesite-attribute.https.html
  │   ▶ FAIL [expected PASS] In embedded cross-site contexts, partitioned cookies can only be set with explicit SameSite=None
  │   │   → assert_equals: expected 0 but got 3
```
https://github.com/servo/servo/actions/runs/26696539019/job/78682355309