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

docs/ChatSession/chat-session-openapi.yaml

@@ -433,6 +433,7 @@ components:
           oneOf:
             - $ref: '#/components/schemas/RunStatusPayload'
             - $ref: '#/components/schemas/AgentMessagePayload'
+            - $ref: '#/components/schemas/MessageDeltaPayload'
             - $ref: '#/components/schemas/RunCancelPayload'
             - $ref: '#/components/schemas/RunErrorPayload'
             - type: object
@@ -506,6 +507,14 @@ components:
           format: uuid
         provider:
           type: string
+    MessageDeltaPayload:
+      type: object
+      properties:
+        run_id:
+          type: string
+          format: uuid
+        delta_index:
+          type: integer
     RunCancelPayload:
       type: object
       properties: