test(crawlers): empty/missing path early-returns for NpmCrawler

Three tests covering find_by_purls with empty PURL list, nonexistent
node_modules, and crawl_all with no packages installed.

Assisted-by: Claude Code:claude-opus-4-7

Author: mikolalysenko

crates/socket-patch-core/tests/crawlers_empty_paths_e2e.rs

@@ -0,0 +1,56 @@
+//! Integration coverage for the crawlers' empty/missing-path early
+//! returns. Each crawler's `find_by_purls` and `crawl_all` short-
+//! circuits when the discovery root doesn't exist or no PURLs match
+//! its scheme — branches the apply-CLI suite doesn't naturally
+//! exercise because those tests always pre-stage a layout.
+
+use socket_patch_core::crawlers::types::CrawlerOptions;
+use socket_patch_core::crawlers::NpmCrawler;
+use std::path::PathBuf;
+
+fn options_at(root: &std::path::Path) -> CrawlerOptions {
+    CrawlerOptions {
+        cwd: root.to_path_buf(),
+        global: false,
+        global_prefix: None,
+        batch_size: 100,
+    }
+}
+
+#[tokio::test]
+async fn npm_crawler_find_by_purls_with_empty_purls_returns_empty_map() {
+    let tmp = tempfile::tempdir().unwrap();
+    let crawler = NpmCrawler;
+    let result = crawler
+        .find_by_purls(tmp.path(), &[])
+        .await
+        .unwrap();
+    assert!(result.is_empty(), "empty PURL list → empty result");
+}
+
+#[tokio::test]
+async fn npm_crawler_find_by_purls_with_nonexistent_node_modules_returns_empty() {
+    let tmp = tempfile::tempdir().unwrap();
+    let nonexistent = tmp.path().join("missing_node_modules");
+    let crawler = NpmCrawler;
+    let result = crawler
+        .find_by_purls(
+            &nonexistent,
+            &["pkg:npm/lodash@4.17.21".to_string()],
+        )
+        .await
+        .unwrap();
+    assert!(result.is_empty(), "nonexistent node_modules → empty");
+}
+
+#[tokio::test]
+async fn npm_crawler_crawl_all_with_no_packages_returns_empty() {
+    let tmp = tempfile::tempdir().unwrap();
+    let crawler = NpmCrawler;
+    let result = crawler.crawl_all(&options_at(tmp.path())).await;
+    assert!(result.is_empty(), "no packages installed → empty crawl");
+}
+
+// Marker import suppress.
+#[allow(dead_code)]
+fn _path_marker(_p: PathBuf) {}