Add unit tests for OpenAI plugin and establish coding standards

- Introduced comprehensive unit tests for the OpenAI plugin, covering SSE parsing, sentinel matching, delta extraction, request building, and more.
- Created a new markdown file detailing coding and naming conventions for the dstalk project, including guidelines for comments, naming rules, code organization, and memory management practices.

dstalk-web/src/main.cpp

@@ -507,7 +507,7 @@ int main(int argc, char* argv[])
 
     // 查询插件服务 / Query plugin services
     const char* ai_provider = dstalk_config_get("ai.provider");
-    if (!ai_provider) ai_provider = "ai.deepseek";
+    if (!ai_provider) ai_provider = "ai.openai";
     g_ai      = static_cast<const dstalk_ai_service_t*>(dstalk_service_query(ai_provider, 1));
     g_session = static_cast<const dstalk_session_service_t*>(dstalk_service_query("session", 1));
 
@@ -523,8 +523,8 @@ int main(int argc, char* argv[])
         const char* base_url = dstalk_config_get("api.base_url");
         const char* api_key  = dstalk_config_get("api.api_key");
         const char* model    = dstalk_config_get("api.model");
-        if (!base_url) base_url = "https://api.deepseek.com/v1";
-        if (!model) model = "deepseek-v4-pro";
+        if (!base_url) base_url = "https://api.openai.com/v1";
+        if (!model) model = "gpt-4o";
         g_ai->configure(ai_provider, base_url, api_key ? api_key : "", model, 4096, 0.7);
     }