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/meta.rs

@@ -512,14 +512,14 @@ use crate::handlers::utils::*;
 #[cfg(test)]
 mod tests {
     use super::*;
-    use tempfile::tempdir;
     use serde_json::json;
+    use tempfile::tempdir;
 
     #[tokio::test]
     async fn test_log_error_fix() {
         let dir = tempdir().unwrap();
         let state = Arc::new(MemoryState::new(dir.path().to_str().unwrap()));
-        
+
         let handler = LogErrorFixHandler;
         let args = json!({
             "signature": "IndexOutOfBounds",
@@ -528,7 +528,7 @@ mod tests {
             "git_commit": "abcdef",
             "git_branch": "main"
         });
-        
+
         let res = handler.execute(args, state.clone()).await.unwrap();
         assert!(res.contains("Error fix logged"));
     }
@@ -537,10 +537,10 @@ mod tests {
     async fn test_project_health() {
         let dir = tempdir().unwrap();
         let state = Arc::new(MemoryState::new(dir.path().to_str().unwrap()));
-        
+
         let handler = GetProjectHealthHandler;
         let args = json!({"namespace": "global"});
-        
+
         let res = handler.execute(args, state.clone()).await.unwrap();
         assert!(res.contains("unresolved_tech_debt"));
     }
@@ -549,7 +549,7 @@ mod tests {
     async fn test_log_decision_and_tech_debt() {
         let dir = tempdir().unwrap();
         let state = Arc::new(MemoryState::new(dir.path().to_str().unwrap()));
-        
+
         let decision_handler = LogDecisionHandler;
         let args_dec = json!({
             "title": "Architecture",
@@ -557,7 +557,10 @@ mod tests {
             "decision": "Use SQLite",
             "consequence": "Simple",
         });
-        let res1 = decision_handler.execute(args_dec, state.clone()).await.unwrap();
+        let res1 = decision_handler
+            .execute(args_dec, state.clone())
+            .await
+            .unwrap();
         assert_eq!(res1, "Decision logged as ADR-0001");
 
         let debt_handler = LogTechDebtHandler;
@@ -571,11 +574,20 @@ mod tests {
             "git_branch": "main",
             "namespace": "global"
         });
-        let res2 = debt_handler.execute(args_debt, state.clone()).await.unwrap();
+        let res2 = debt_handler
+            .execute(args_debt, state.clone())
+            .await
+            .unwrap();
         assert_eq!(res2, "Tech debt logged");
 
         let list_debt = ListTechDebtHandler;
-        let res3 = list_debt.execute(json!({"namespace": "global", "include_resolved": false}), state.clone()).await.unwrap();
+        let res3 = list_debt
+            .execute(
+                json!({"namespace": "global", "include_resolved": false}),
+                state.clone(),
+            )
+            .await
+            .unwrap();
         assert!(res3.contains("Hardcoded path"));
 
         let pref_handler = LearnPreferenceHandler;
@@ -583,7 +595,10 @@ mod tests {
             "key": "formatting",
             "value": "use spaces",
         });
-        let res4 = pref_handler.execute(args_pref, state.clone()).await.unwrap();
+        let res4 = pref_handler
+            .execute(args_pref, state.clone())
+            .await
+            .unwrap();
         assert_eq!(res4, "Preference learned");
 
         let read_pref = ReadPreferencesHandler;
@@ -603,10 +618,16 @@ mod tests {
             "git_commit": "def",
             "git_branch": "main"
         });
-        code_handler.execute(args_code, state.clone()).await.unwrap();
+        code_handler
+            .execute(args_code, state.clone())
+            .await
+            .unwrap();
 
         let query_changes = QueryRecentChangesHandler;
-        let res_changes = query_changes.execute(json!({}), state.clone()).await.unwrap();
+        let res_changes = query_changes
+            .execute(json!({}), state.clone())
+            .await
+            .unwrap();
         assert!(res_changes.contains("main.rs"));
 
         let debt_handler = LogTechDebtHandler;
@@ -620,15 +641,27 @@ mod tests {
             "git_branch": "main",
             "namespace": "global"
         });
-        debt_handler.execute(args_debt, state.clone()).await.unwrap();
+        debt_handler
+            .execute(args_debt, state.clone())
+            .await
+            .unwrap();
 
         // resolve it
         let list_debt = ListTechDebtHandler;
-        let debt_list = list_debt.execute(json!({"namespace": "global", "include_resolved": false}), state.clone()).await.unwrap();
+        let debt_list = list_debt
+            .execute(
+                json!({"namespace": "global", "include_resolved": false}),
+                state.clone(),
+            )
+            .await
+            .unwrap();
         let uuid_start = debt_list.find("id\":\"").unwrap() + 5;
-        let uuid = &debt_list[uuid_start..uuid_start+36];
-        
+        let uuid = &debt_list[uuid_start..uuid_start + 36];
+
         let resolve_debt = ResolveTechDebtHandler;
-        resolve_debt.execute(json!({"id": uuid}), state.clone()).await.unwrap();
+        resolve_debt
+            .execute(json!({"id": uuid}), state.clone())
+            .await
+            .unwrap();
     }
 }