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-core/include/dstalk/dstalk_services.h

@@ -15,7 +15,7 @@ extern "C" {
 #endif
 
 /* ---- AI 服务 vtable / AI service vtable ---- */
-/* 以名称如 "ai.deepseek" 或 "ai.anthropic" 注册 / Registered under names such as "ai.deepseek" or "ai.anthropic" */
+/* 以名称如 "ai.openai" 或 "ai.anthropic" 注册 / Registered under names such as "ai.openai" or "ai.anthropic" */
 typedef struct {
     /* 配置服务商连接 (base_url, api_key, model 等) / Configure provider connection (base_url, api_key, model, etc.) */
     int (*configure)(const char* provider, const char* base_url,