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/internal/lsp/client_test.go

@@ -26,6 +26,7 @@ func TestNormalizeURIRebasesRelativeFileURI(t *testing.T) {
 	}
 }
 
+// 绝对 file URI 不应被重写。
 func TestNormalizeURIKeepsAbsoluteFileURI(t *testing.T) {
 	workspace, err := filepath.Abs(filepath.Join("testdata", "ws"))
 	if err != nil {