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.

dstalk_core/include/dstalk/dstalk_services.h

@@ -35,6 +35,42 @@ typedef struct {
     void (*free_result)(dstalk_chat_result_t* result);
 } dstalk_ai_service_t;
 
+/* ---- AI endpoint manager 服务 vtable / AI endpoint manager service vtable ---- */
+/* 以服务名称 "ai_endpoint_mgr" 注册；用于按名称路由到多个 provider/model endpoint。 / Registered as "ai_endpoint_mgr"; routes named endpoints to provider/model configs. */
+typedef struct {
+    /* 返回已配置 endpoint 数量 / Return configured endpoint count. */
+    int (*count)(void);
+    /* 返回 endpoint 列表 JSON，调用方用 dstalk_free 释放 / Return endpoint list JSON; caller frees with dstalk_free. */
+    char* (*list_json)(void);
+    /* 获取当前 active endpoint 名称 / Get current active endpoint name.
+       返回的指针指向 thread_local 存储区，调用方不需要释放。该指针在同线程下一次
+       get_active 调用前或 active endpoint 状态变化前有效；跨线程并发调用各自拥有
+       独立的 thread_local 副本，互不干扰。
+       The returned pointer points to thread-local storage; caller must not free it.
+       Valid until the next get_active call on the same thread or until the active
+       endpoint changes. Concurrent calls from different threads each have their own
+       independent thread-local copy. */
+    const char* (*get_active)(void);
+    /* 设置当前 active endpoint / Set current active endpoint. */
+    int (*set_active)(const char* endpoint_name);
+    /* 设置 endpoint 模型名；endpoint_name 为空时修改 active endpoint / Set endpoint model; null endpoint_name updates active endpoint. */
+    int (*set_model)(const char* endpoint_name, const char* model);
+    /* 在指定 endpoint 上执行阻塞 chat / Run blocking chat on a named endpoint. */
+    dstalk_chat_result_t (*chat)(
+        const char* endpoint_name,
+        const dstalk_message_t* history, int history_len,
+        const char* user_input,
+        const char* tools_json);
+    /* 在指定 endpoint 上执行流式 chat / Run streaming chat on a named endpoint. */
+    dstalk_chat_result_t (*chat_stream)(
+        const char* endpoint_name,
+        const dstalk_message_t* history, int history_len,
+        const char* user_input,
+        dstalk_stream_cb cb, void* userdata);
+    /* 释放 endpoint manager 返回的 chat result / Free chat result returned by endpoint manager. */
+    void (*free_result)(dstalk_chat_result_t* result);
+} dstalk_ai_endpoint_mgr_t;
+
 /* ---- 会话服务 vtable / Session service vtable ---- */
 /* 以服务名称 "session" 注册 / Registered under service name "session" */
 typedef struct {