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/mcp.rs

@@ -36,7 +36,7 @@ mod tests {
     use super::*;
     use schemars::JsonSchema;
     use serde::Serialize;
-    
+
     #[derive(JsonSchema, Serialize)]
     struct DummyTool {
         name: String,
@@ -66,11 +66,10 @@ mod tests {
         assert_eq!(def["name"], "dummy");
         assert_eq!(def["description"], "A dummy tool");
         assert!(def["inputSchema"].is_object());
-        
+
         let schema = def["inputSchema"].as_object().unwrap();
         let properties = schema["properties"].as_object().unwrap();
         assert!(properties.contains_key("name"));
         assert!(properties.contains_key("age"));
     }
 }
-