feat: add AI endpoint manager plugin with configuration and routing capabilities

- Introduced `ai_endpoint_mgr` plugin to manage multiple AI provider endpoints.
- Added configuration reference documentation for `config.toml`.
- Implemented endpoint loading, active endpoint switching, and model mutation.
- Included error handling for missing endpoints and configuration failures.
- Developed unit tests covering various scenarios including error paths and concurrency.

plugins_upper/openai/src/openai_plugin.cpp

@@ -49,6 +49,7 @@ static std::string build_headers_json(const std::string& auth_header_value)
 static void append_history(json::array& msgs,
                            const dstalk_message_t* history, int history_len)
 {
+    if (!history) return; // 防御: history 为空时直接返回 / Defensive: return early when history is null
     for (int i = 0; i < history_len; ++i) {
         const auto& m = history[i];
         json::object obj;