Linter followup (#868)

Expanded Ruff rules and completed fixes

tests/core/test_image_cache.py

@@ -0,0 +1,22 @@
+"""Tests for targeted image cache safety and fetch fallbacks."""
+
+import requests
+
+from shelfmark.core.image_cache import ImageCacheService
+
+
+def test_is_safe_url_rejects_invalid_ipv6_url() -> None:
+    assert ImageCacheService._is_safe_url("http://[") is False
+
+
+def test_fetch_and_cache_returns_none_on_request_exception(tmp_path, monkeypatch) -> None:
+    cache = ImageCacheService(tmp_path)
+    monkeypatch.setattr(cache, "_is_safe_url", lambda _url: True)
+
+    def fake_get(*args, **kwargs):
+        raise requests.exceptions.TooManyRedirects("too many redirects")
+
+    monkeypatch.setattr("shelfmark.core.image_cache.requests.get", fake_get)
+
+    assert cache.fetch_and_cache("cover-1", "https://example.com/cover.jpg") is None
+    assert "cover-1" not in cache._index