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/refresh.go

@@ -53,7 +53,11 @@ func RefreshModuleIngest(ctx context.Context, st store.Backend, hc *http.Client,
 		return fmt.Errorf("module disabled")
 	}
 
-	rows, err := collectModulePrefixRows(ctx, st, hc, tenantID, mod, nil)
+	var prior []store.PrefixRow
+	if snap, ok, _ := st.GetModulePrefixSnapshot(tenantID, moduleID); ok && snap != nil {
+		prior = snap.Prefixes
+	}
+	rows, err := collectModulePrefixRows(ctx, st, hc, tenantID, mod, prior)
 	if err != nil {
 		return err
 	}