refactor: apply rust best practices and fix memory optimizations

- Optimized memory allocation in router.rs by offloading JSON serialization to spawn_blocking and using references.
- Prevented full graph duplication on startup in state.rs index rebuild.
- Eliminated massive String allocations in QueryGraphPathHandler BFS loops.
- Avoided temporary Strings in VisualizeGraphHandler via inline writing.
- Fixed O(N) full-graph deduplication in MergeEntitiesHandler to scale efficiently.

server/src/handlers/env.rs

@@ -164,14 +164,14 @@ impl McpTool for GetEnvironmentDetailsHandler {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use tempfile::tempdir;
     use serde_json::json;
+    use tempfile::tempdir;
 
     #[tokio::test]
     async fn test_env_fingerprint() {
         let dir = tempdir().unwrap();
         let state = Arc::new(MemoryState::new(dir.path().to_str().unwrap()));
-        
+
         let update_handler = UpdateEnvFingerprintHandler;
         let args = json!({
             "namespace": "global",
@@ -179,12 +179,15 @@ mod tests {
                 "rustc": "1.70.0"
             }
         });
-        
+
         let res = update_handler.execute(args, state.clone()).await.unwrap();
         assert_eq!(res, "Env fingerprint updated");
 
         let read_handler = ReadEnvFingerprintHandler;
-        let res2 = read_handler.execute(json!({"namespace": "global"}), state.clone()).await.unwrap();
+        let res2 = read_handler
+            .execute(json!({"namespace": "global"}), state.clone())
+            .await
+            .unwrap();
         assert!(res2.contains("rustc"));
         assert!(res2.contains("1.70.0"));
     }
@@ -193,7 +196,7 @@ mod tests {
     async fn test_env_details() {
         let dir = tempdir().unwrap();
         let state = Arc::new(MemoryState::new(dir.path().to_str().unwrap()));
-        
+
         // Ensure namespace is present in test setup
         state.environments.modify(|e| {
             e.push(crate::models::EnvironmentDetail {
@@ -207,8 +210,11 @@ mod tests {
         });
 
         let handler = GetEnvironmentDetailsHandler;
-        let res = handler.execute(json!({"namespace": "global"}), state.clone()).await.unwrap();
-        
+        let res = handler
+            .execute(json!({"namespace": "global"}), state.clone())
+            .await
+            .unwrap();
+
         assert!(res.contains("global"));
     }
 
@@ -241,8 +247,11 @@ mod tests {
         assert_eq!(res2, "Environment registered");
 
         let get_handler = GetEnvironmentDetailsHandler;
-        let res3 = get_handler.execute(json!({"namespace": "global"}), state.clone()).await.unwrap();
+        let res3 = get_handler
+            .execute(json!({"namespace": "global"}), state.clone())
+            .await
+            .unwrap();
         assert!(res3.contains("prod.local"));
-        assert!(res3.len() > 0);
+        assert!(!res3.is_empty());
     }
 }