refactor: WebSocket Factory 패턴 제거 및 nullability 경고 해결

- WebSocketClientConnection에 생성자 추가하여 Factory 패턴 제거
- WebSocketClientConnectionFactory 및 IClientConnectionFactory 삭제
- WebSocketMiddleware에서 직접 생성자 사용하도록 변경
- EnvironmentVariableConfigurationProvider nullability 경고 해결
- ErrorResponse 속성 초기화로 nullability 경고 해결

Author: ImGdevel

ProjectVG.Api/Configuration/EnvironmentVariableConfigurationProvider.cs

@@ -14,15 +14,15 @@ public EnvironmentVariableConfigurationProvider(IConfiguration configuration)
 
         public override void Load()
         {
-            var data = new Dictionary<string, string>();
+            var data = new Dictionary<string, string?>();
 
             // 모든 설정을 순회하면서 환경 변수 참조를 찾아서 치환
             ReplaceEnvironmentVariables(_configuration, data, "");
 
             Data = data;
         }
 
-        private void ReplaceEnvironmentVariables(IConfiguration configuration, Dictionary<string, string> data, string prefix)
+        private void ReplaceEnvironmentVariables(IConfiguration configuration, Dictionary<string, string?> data, string prefix)
         {
             foreach (var child in configuration.GetChildren())
             {

ProjectVG.Common/Models/ErrorResponse.cs

@@ -2,8 +2,8 @@ namespace ProjectVG.Common.Models
 {
     public class ErrorResponse
     {
-        public string ErrorCode { get; set; }
-        public string Message { get; set; }
+        public string ErrorCode { get; set; } = string.Empty;
+        public string Message { get; set; } = string.Empty;
         public int StatusCode { get; set; }
         public DateTime Timestamp { get; set; }
         public string? TraceId { get; set; }