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/completion/service_test.go

@@ -52,6 +52,7 @@ func (f *fakeLSPClient) Completion(_ context.Context, uri string, line, characte
 	return f.completionResp, nil
 }
 
+// 首次请求应发送 didOpen，并继续请求 completion。
 func TestServiceCompleteFirstRequestSendsDidOpen(t *testing.T) {
 	fake := &fakeLSPClient{
 		completionResp: Response{Items: []Item{{Label: "Println"}}, IsIncomplete: true},
@@ -84,6 +85,7 @@ func TestServiceCompleteFirstRequestSendsDidOpen(t *testing.T) {
 	}
 }
 
+// 同一文档第二次请求应发送 didChange，且版本号递增。
 func TestServiceCompleteSecondRequestUsesDidChange(t *testing.T) {
 	fake := &fakeLSPClient{}
 	svc := NewService(fake)
@@ -119,6 +121,7 @@ func TestServiceCompleteSecondRequestUsesDidChange(t *testing.T) {
 	}
 }
 
+// 参数不合法时应直接返回 ErrInvalidRequest。
 func TestServiceCompleteValidatesRequest(t *testing.T) {
 	svc := NewService(&fakeLSPClient{})
 
@@ -128,6 +131,7 @@ func TestServiceCompleteValidatesRequest(t *testing.T) {
 	}
 }
 
+// 底层 client 出错应向上透传错误。
 func TestServiceCompleteReturnsClientError(t *testing.T) {
 	fake := &fakeLSPClient{openErr: errors.New("open failed")}
 	svc := NewService(fake)