fix: address PR review comments for Image provider

- Extract _is_registry_url() into ImageProvider to deduplicate logic
- Restore relative URL handling in _next_page_url for path-only Link headers
- Yield empty findings on generic error in scan_per_image
- Add credential validation to ImageProviderSecret serializer

tests/providers/image/image_provider_test.py

@@ -697,6 +697,35 @@ class TestExtractRegistry:
         assert ImageProvider._extract_registry("nginx") is None
 
 
+class TestIsRegistryUrl:
+    def test_registry_url_with_namespace(self):
+        assert ImageProvider._is_registry_url("docker.io/andoniaf") is True
+
+    def test_registry_url_ghcr(self):
+        assert ImageProvider._is_registry_url("ghcr.io/org") is True
+
+    def test_image_ref_with_tag(self):
+        assert ImageProvider._is_registry_url("ghcr.io/user/image:tag") is False
+
+    def test_image_ref_with_repo(self):
+        assert ImageProvider._is_registry_url("ghcr.io/user/image") is False
+
+    def test_dockerhub_short_image(self):
+        assert ImageProvider._is_registry_url("alpine:3.18") is False
+
+    def test_dockerhub_with_namespace(self):
+        assert ImageProvider._is_registry_url("andoniaf/test:tag") is False
+
+    def test_bare_image_name(self):
+        assert ImageProvider._is_registry_url("nginx") is False
+
+    def test_localhost_namespace(self):
+        assert ImageProvider._is_registry_url("localhost:5000/myns") is True
+
+    def test_localhost_image_with_tag(self):
+        assert ImageProvider._is_registry_url("localhost:5000/myns/image:v1") is False
+
+
 class TestCleanup:
     def test_cleanup_idempotent(self):
         """Test cleanup is safe to call multiple times."""

tests/providers/image/lib/registry/test_oci_adapter.py

@@ -300,6 +300,16 @@ class TestOciAdapterNextPageUrl:
             == "https://reg.io/v2/_catalog?n=200&last=b"
         )
 
+    def test_link_header_relative_url(self):
+        resp = MagicMock(
+            headers={"Link": '</v2/_catalog?n=200&last=b>; rel="next"'},
+            url="https://reg.io/v2/_catalog?n=200",
+        )
+        assert (
+            OciRegistryAdapter._next_page_url(resp)
+            == "https://reg.io/v2/_catalog?n=200&last=b"
+        )
+
     def test_link_header_no_next(self):
         resp = MagicMock(
             headers={"Link": '<https://reg.io/v2/_catalog?n=200>; rel="prev"'}