test(terraphim_dsm): add missing unit tests for KnowledgeGraph

Add two tests that satisfy the remaining acceptance criteria from #2899:
- test_new_creates_empty_graph: explicitly asserts KnowledgeGraph::new()
  starts with 0 concepts (first AC was unaddressed by the existing 3 tests)
- test_group_by_concept_buckets_modules: verifies group_by_concept correctly
  partitions matched vs uncategorized modules

cargo test -p terraphim_dsm now reports 5 passed / 0 failed.

Refs #2899

Author: Test User

crates/terraphim_dsm/src/knowledge.rs

@@ -187,6 +187,13 @@ mod tests {
     use std::io::Write;
     use tempfile::TempDir;
 
+    #[test]
+    fn test_new_creates_empty_graph() {
+        let temp_dir = TempDir::new().unwrap();
+        let kg = KnowledgeGraph::new(temp_dir.path().to_path_buf());
+        assert_eq!(kg.concept_count(), 0);
+    }
+
     #[test]
     fn test_knowledge_graph_loading() {
         let temp_dir = TempDir::new().unwrap();
@@ -257,4 +264,36 @@ mod tests {
         assert!(kg.are_related("auth_module", "security_handler"));
         assert!(!kg.are_related("auth_module", "ui_component"));
     }
+
+    #[test]
+    fn test_group_by_concept_buckets_modules() {
+        let temp_dir = TempDir::new().unwrap();
+        let kg_dir = temp_dir.path().join("kg");
+        std::fs::create_dir(&kg_dir).unwrap();
+
+        let mut auth_file = std::fs::File::create(kg_dir.join("Authentication.md")).unwrap();
+        writeln!(auth_file, "# Authentication").unwrap();
+        writeln!(auth_file, "synonyms:: auth, login").unwrap();
+
+        let mut kg = KnowledgeGraph::new(kg_dir.clone());
+        kg.load_from_directory(&kg_dir).unwrap();
+
+        let modules = vec![
+            "service::auth_handler".to_string(),
+            "service::login_manager".to_string(),
+            "service::unrelated_utils".to_string(),
+        ];
+        let groups = kg.group_by_concept(&modules);
+
+        assert!(
+            groups.contains_key("Authentication"),
+            "auth modules must be grouped"
+        );
+        assert_eq!(groups["Authentication"].len(), 2);
+        assert!(
+            groups.contains_key("uncategorized"),
+            "unmatched modules go to uncategorized"
+        );
+        assert_eq!(groups["uncategorized"].len(), 1);
+    }
 }