test: add CDN prefetch tests for 304 responses and snapshot handling

- Introduced `TestRefreshModuleIngest_CDN304UsesStoredSnapshot` to verify that the module ingest correctly uses stored snapshots when receiving a 304 Not Modified response.
- Added `TestCollectModulePrefixRows_CDN304RetriesWithoutETag` to ensure that the system retries without the If-None-Match header after a 304 response, and correctly collects prefixes on subsequent successful requests.
- Updated `cachedCDNPrefixRows` function to improve handling of cached prefixes during CDN source processing.
- Modified `RefreshModuleIngest` to utilize prior snapshots when collecting module prefix rows.

internal/pipeline/cdn_prefetch_test.go

@@ -97,3 +97,96 @@ func TestCollectModulePrefixRows_CDN304UsesSnapshot(t *testing.T) {
 		t.Fatalf("want cached prefix on 304, got %+v", collected)
 	}
 }
+
+func TestRefreshModuleIngest_CDN304UsesStoredSnapshot(t *testing.T) {
+	m := store.NewMemory()
+	m.SeedDemo()
+	tenant, _, _, _, _ := m.DemoIDs()
+
+	mod, err := m.CreateModule(tenant, &store.Module{
+		Type:    "CDN_CIDRS",
+		Name:    "cdn-304-ingest",
+		Enabled: true,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusNotModified)
+	}))
+	defer srv.Close()
+
+	etag := "etag-stable"
+	src, err := m.CreateCDNSource(tenant, mod.ID, &store.CDNSource{
+		SourceKind: "txt",
+		URL:        srv.URL,
+		Etag:       etag,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	prior := []store.PrefixRow{{
+		Prefix: "203.0.113.0/24",
+		Source: cdnSourceKey(src.ID),
+	}}
+	if err := mergeCDNSourceIntoModuleSnapshot(m, tenant, mod, src.ID, prior); err != nil {
+		t.Fatal(err)
+	}
+
+	if err := RefreshModuleIngest(context.Background(), m, srv.Client(), tenant, mod.ID); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestCollectModulePrefixRows_CDN304RetriesWithoutETag(t *testing.T) {
+	m := store.NewMemory()
+	m.SeedDemo()
+	tenant, _, _, _, _ := m.DemoIDs()
+
+	mod, err := m.CreateModule(tenant, &store.Module{
+		Type:    "CDN_CIDRS",
+		Name:    "cdn-304-retry",
+		Enabled: true,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var calls int
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		calls++
+		if calls == 1 {
+			if got := strings.TrimSpace(r.Header.Get("If-None-Match")); got != "etag-stable" {
+				t.Fatalf("first request want If-None-Match etag-stable, got %q", got)
+			}
+			w.WriteHeader(http.StatusNotModified)
+			return
+		}
+		if got := strings.TrimSpace(r.Header.Get("If-None-Match")); got != "" {
+			t.Fatalf("retry must omit If-None-Match, got %q", got)
+		}
+		w.Header().Set("ETag", "etag-stable")
+		_, _ = w.Write([]byte("198.51.100.0/24\n"))
+	}))
+	defer srv.Close()
+
+	if _, err := m.CreateCDNSource(tenant, mod.ID, &store.CDNSource{
+		SourceKind: "txt",
+		URL:        srv.URL,
+		Etag:       "etag-stable",
+	}); err != nil {
+		t.Fatal(err)
+	}
+
+	collected, err := collectModulePrefixRows(context.Background(), m, srv.Client(), tenant, mod, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if calls != 2 {
+		t.Fatalf("want 2 HTTP calls (304 then 200), got %d", calls)
+	}
+	if len(collected) != 1 || collected[0].Prefix != "198.51.100.0/24" {
+		t.Fatalf("unexpected collected rows: %+v", collected)
+	}
+}