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/anthropic/src/anthropic_plugin.cpp

@@ -65,6 +65,7 @@ static std::string build_request_json(
     std::string system_prompt;
     json::array msgs;
 
+    if (history) { // 防御: history 为空时跳过历史遍历 / Defensive: skip history iteration when null
     for (int i = 0; i < history_len; ++i) {
         const auto& m = history[i];
         if (m.role && std::strcmp(m.role, "system") == 0) {
@@ -77,6 +78,7 @@ static std::string build_request_json(
         obj["content"] = m.content ? m.content : "";
         msgs.push_back(obj);
     }
+    } // if (history)
 
     // 追加当前用户输入 / Append current user input
     {