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/api/ws.rs

@@ -146,10 +146,10 @@ pub async fn handle_socket(socket: WebSocket, state: Arc<AppState>, _client_type
 #[cfg(test)]
 mod tests {
     use super::*;
-    use std::sync::atomic::AtomicUsize;
-    use std::sync::RwLock;
     use crate::router::MemoryHandler;
     use crate::state::MemoryState;
+    use std::sync::RwLock;
+    use std::sync::atomic::AtomicUsize;
     use tempfile::tempdir;
 
     #[tokio::test]
@@ -163,7 +163,11 @@ mod tests {
         });
 
         // Insert a dummy client
-        app_state.clients.write().unwrap().insert("test-session".to_string(), tokio::sync::mpsc::channel(1).0);
+        app_state
+            .clients
+            .write()
+            .unwrap()
+            .insert("test-session".to_string(), tokio::sync::mpsc::channel(1).0);
 
         let send_task = tokio::spawn(async {});
         let recv_task = tokio::spawn(async {});