main: 引入 AgentProvider 流式事件与 OpenAI 兼容适配

- 增加流式事件流支持，Provider 输出 `message.delta` 等事件
- 实现 OpenAI 兼容适配器，包括 RequestBuilder、ApiClient 等模块
- 更新 Agent Run 逻辑，支持流式增量写入与模型完成状态管理
- 扩展配置项 `agent.openai.*`，支持模型、密钥等配置
- 优化文档，完善流式事件与消息类型说明
- 增加单元测试，覆盖 Provider 和 OpenAI 适配相关逻辑
- 更新环境变量与配置示例，支持新功能

app/Services/ContextBuilder.php

@@ -3,6 +3,7 @@
 namespace App\Services;
 
 use App\Models\Message;
+use App\Services\Agent\AgentContext;
 use Illuminate\Support\Collection;
 
 class ContextBuilder
@@ -11,18 +12,15 @@ class ContextBuilder
     {
     }
 
-    /**
-     * @return array<string, mixed>
-     */
-    public function build(string $sessionId, string $runId): array
+    public function build(string $sessionId, string $runId): AgentContext
     {
         $messages = $this->loadRecentMessages($sessionId);
 
-        return [
-            'run_id' => $runId,
-            'session_id' => $sessionId,
-            'system_prompt' => 'You are an agent inside ARS. Respond concisely in plain text.',
-            'messages' => $messages->map(function (Message $message) {
+        return new AgentContext(
+            $runId,
+            $sessionId,
+            'You are an agent inside ARS. Respond concisely in plain text.',
+            $messages->map(function (Message $message) {
                 return [
                     'message_id' => $message->message_id,
                     'role' => $message->role,
@@ -30,8 +28,8 @@ class ContextBuilder
                     'content' => $message->content,
                     'seq' => $message->seq,
                 ];
-            })->values()->all(),
-        ];
+            })->values()->all()
+        );
     }
 
     private function loadRecentMessages(string $sessionId): Collection