build(ci): harden preview perf gate

code/tests/test_preview_perf_fetch_retry.py

@@ -0,0 +1,20 @@
+from code.scripts import preview_perf_benchmark as perf
+
+
+def test_fetch_all_theme_slugs_retries(monkeypatch):
+    calls = {"count": 0}
+
+    def fake_fetch(url):  # type: ignore[override]
+        calls["count"] += 1
+        if calls["count"] == 1:
+            raise RuntimeError("transient 500")
+        assert url.endswith("offset=0")
+        return {"items": [{"id": "alpha"}], "next_offset": None}
+
+    monkeypatch.setattr(perf, "_fetch_json", fake_fetch)
+    monkeypatch.setattr(perf.time, "sleep", lambda *_args, **_kwargs: None)
+
+    slugs = perf.fetch_all_theme_slugs("http://example.com", page_limit=1)
+
+    assert slugs == ["alpha"]
+    assert calls["count"] == 2