feat: enhance API and session management with Nacos and Redis integration

- Add Nacos registry for service registration and deregistration.
- Implement Redis registry for session management with heartbeat and session claiming.
- Improve completion service with session handling and request validation.
- Enhance WebSocket handling for completion requests with JSON-RPC support.
- Add tests for new registry implementations and completion manager functionalities.
- Refactor existing code for better readability and maintainability.

backend/config.json

@@ -0,0 +1,55 @@
+{
+  "port": "8080",
+  "workspaceDir": ".",
+  "allowOrigin": "*",
+  "apiToken": "",
+  "requestTimeout": "10s",
+  "maxBodyBytes": 2097152,
+  "sessionTTL": "20m",
+  "cleanupInterval": "2m",
+  "maxSessions": 256,
+  "instanceID": "",
+  "instanceURL": "http://127.0.0.1:8080",
+  "enableRedis": true,
+  "redisAddr": "10.0.0.10:6379",
+  "redisPassword": "",
+  "redisDB": 1,
+  "redisKeyPrefix": "lsp-gateway",
+  "instanceTTL": "30s",
+  "heartbeat": "10s",
+  "enableNacosRegister": true,
+  "nacosServerAddr": "10.0.0.10:8848",
+  "nacosNamespace": "",
+  "nacosGroup": "DEFAULT_GROUP",
+  "nacosServiceName": "lsp-gateway",
+  "nacosClusterName": "",
+  "nacosUsername": "",
+  "nacosPassword": "",
+  "nacosRegisterIP": "",
+  "nacosRegisterPort": 8080,
+  "nacosEphemeral": true,
+  "servers": [
+    {
+      "language": "go",
+      "languageId": "go",
+      "command": "gopls",
+      "args": []
+    },
+    {
+      "language": "javascript",
+      "languageId": "javascript",
+      "command": "typescript-language-server",
+      "args": [
+        "--stdio"
+      ]
+    },
+    {
+      "language": "typescript",
+      "languageId": "typescript",
+      "command": "typescript-language-server",
+      "args": [
+        "--stdio"
+      ]
+    }
+  ]
+}