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/manager_test.go

@@ -31,6 +31,7 @@ func (f *fakeRuntimeClient) Close() error {
 	return nil
 }
 
+// 同 language+session 的请求应复用同一个底层 client。
 func TestManagerCompleteSelectsLanguageAndSession(t *testing.T) {
 	createCount := 0
 	factory := func(_ context.Context, spec LanguageServerSpec, _ string) (RuntimeClient, error) {
@@ -68,6 +69,7 @@ func TestManagerCompleteSelectsLanguageAndSession(t *testing.T) {
 	}
 }
 
+// 未配置的语言应返回 ErrUnsupportedLanguage。
 func TestManagerCompleteUnsupportedLanguage(t *testing.T) {
 	m := NewManager(ManagerConfig{}, []LanguageServerSpec{
 		{Language: "go", LanguageID: "go", Command: "gopls"},
@@ -88,6 +90,7 @@ func TestManagerCompleteUnsupportedLanguage(t *testing.T) {
 	}
 }
 
+// 超过空闲 TTL 的会话应被后台清理并关闭 client。
 func TestManagerCleanupIdleSession(t *testing.T) {
 	client := &fakeRuntimeClient{}
 	m := NewManager(ManagerConfig{