build(ci): harden preview perf gate startup

code/tests/test_preview_perf_fetch_retry.py

@@ -18,3 +18,22 @@ def test_fetch_all_theme_slugs_retries(monkeypatch):
 
     assert slugs == ["alpha"]
     assert calls["count"] == 2
+
+
+def test_fetch_all_theme_slugs_page_level_retry(monkeypatch):
+    calls = {"count": 0}
+
+    def fake_fetch_with_retry(url, attempts=3, delay=0.6):  # type: ignore[override]
+        calls["count"] += 1
+        if calls["count"] < 3:
+            raise RuntimeError("service warming up")
+        assert url.endswith("offset=0")
+        return {"items": [{"id": "alpha"}], "next_offset": None}
+
+    monkeypatch.setattr(perf, "_fetch_json_with_retry", fake_fetch_with_retry)
+    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"] == 3