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.

tests/context_plugin_test.cpp

@@ -40,10 +40,10 @@ int main()
     {
         std::ofstream config(config_path);
         config << "[api]\n"
-               << "provider = \"deepseek\"\n"
-               << "base_url = \"https://api.deepseek.com/v1\"\n"
+               << "provider = \"openai\"\n"
+               << "base_url = \"https://api.openai.com/v1\"\n"
                << "api_key = \"test-key\"\n"
-               << "model = \"deepseek-v4-pro\"\n";
+               << "model = \"gpt-4o\"\n";
     }
 
     if (dstalk_init(config_path.string().c_str()) != 0) {