feat(openai): 完善 Chat 与 Responses 参数转发

原生 Chat/Responses 改为透明转发，保留标准工具结构并保护调用方显式参数。补齐 Responses 到 Chat 的兼容转换、协议路由边界、完整响应和流式事件，并同步更新 Swagger、回归测试与真实验收脚本。

验证：
- cd apps/api && env -u AI_GATEWAY_TEST_DATABASE_URL go test ./... -count=1
- pnpm openapi
- pnpm lint
- pnpm test
- pnpm build
- gofmt -l 无输出
- git diff --check 通过

风险：
- Chat 回退无法等价表达的 Responses 原生能力现在会返回 unsupported_response_parameter
- 真实供应商 E2E 因本地没有已启用的平台模型候选而未完成

apps/api/docs/swagger.yaml

@@ -154,17 +154,22 @@ definitions:
       index:
         example: 0
         type: integer
+      logprobs: {}
       message:
         $ref: '#/definitions/httpapi.ChatCompletionChoiceMessage'
     type: object
   httpapi.ChatCompletionChoiceMessage:
     properties:
-      content:
-        example: Hello
+      annotations: {}
+      audio: {}
+      content: {}
+      function_call: {}
+      refusal:
         type: string
       role:
         example: assistant
         type: string
+      tool_calls: {}
     type: object
   httpapi.ChatCompletionCompatibleResponse:
     properties:
@@ -184,6 +189,10 @@ definitions:
       object:
         example: chat.completion
         type: string
+      service_tier:
+        type: string
+      system_fingerprint:
+        type: string
       usage:
         $ref: '#/definitions/httpapi.ChatCompletionUsage'
     type: object
@@ -323,10 +332,13 @@ definitions:
     type: object
   httpapi.ChatMessage:
     properties:
+      audio: {}
       content: {}
       function_call: {}
       name:
         type: string
+      refusal:
+        type: string
       role:
         example: user
         type: string
@@ -1195,15 +1207,29 @@ definitions:
     type: object
   httpapi.ResponsesCompatibleResponse:
     properties:
+      background:
+        type: boolean
+      completed_at:
+        type: integer
+      conversation: {}
       created_at:
         example: 1710000000
         type: integer
+      error: {}
       id:
         example: resp_0123456789abcdef0123456789abcdef
         type: string
+      incomplete_details: {}
+      instructions: {}
+      max_output_tokens: {}
+      max_tool_calls: {}
+      metadata:
+        additionalProperties: true
+        type: object
       model:
         example: Doubao Seed 2.0 Pro
         type: string
+      moderation: {}
       object:
         example: response
         type: string
@@ -1215,25 +1241,47 @@ definitions:
       output_text:
         example: Hello
         type: string
-      previous_response_id:
-        example: resp_abcdef0123456789abcdef0123456789
-        type: string
+      parallel_tool_calls:
+        type: boolean
+      previous_response_id: {}
+      prompt: {}
+      prompt_cache_key: {}
+      prompt_cache_options: {}
+      prompt_cache_retention: {}
+      reasoning: {}
+      safety_identifier: {}
+      service_tier: {}
       status:
+        enum:
+        - queued
+        - in_progress
+        - completed
+        - incomplete
+        - failed
+        - cancelled
         example: completed
         type: string
+      temperature: {}
+      text: {}
+      tool_choice: {}
+      tools:
+        items:
+          additionalProperties: true
+          type: object
+        type: array
+      top_logprobs: {}
+      top_p: {}
+      truncation: {}
       usage:
         additionalProperties: true
         type: object
+      user: {}
     type: object
   httpapi.ResponsesRequest:
     properties:
       background:
         type: boolean
-      context_management:
-        items:
-          additionalProperties: true
-          type: object
-        type: array
+      context_management: {}
       conversation: {}
       include:
         items:
@@ -8559,8 +8607,9 @@ paths:
     post:
       consumes:
       - application/json
-      description: /api/v1/chat/completions 同步执行：stream=true 返回 text/event-stream
-        SSE；stream=false 或未传返回兼容 JSON；该接口忽略 X-Async。
+      description: /api/v1/chat/completions 同步执行：除 Gateway 路由字段外透明转发当前及未来 OpenAI 字段与嵌套结构，完整保留
+        function/custom tools、tool_choice、tool_calls 和旧版 function_call；stream=true
+        返回 text/event-stream SSE，stream=false 或未传返回兼容 JSON；该接口忽略 X-Async。
       parameters:
       - description: 该接口忽略此参数
         in: header
@@ -9926,11 +9975,14 @@ paths:
     post:
       consumes:
       - application/json
-      description: 公开 OpenAI-compatible Responses 入口。模型声明 openai_responses 时原生转发，否则使用
-        Chat Completions 转换；store 缺省为 true。previous_response_id 严格绑定首次成功的平台模型和上游协议，链路不可用时不跨平台续接。未提供
-        previous_response_id 时由调用方管理完整状态，Gateway 以本轮 input/messages 为准且不追加本地历史。
+      description: 公开 OpenAI-compatible Responses 入口。模型声明 openai_responses 时除 Gateway
+        路由字段外透明转发当前及未来 OpenAI 字段；否则仅对具有等价 Chat Completions 语义的请求执行转换。background、conversation、prompt
+        template、context management、max_tool_calls、自动 truncation、reasoning summary、内置工具、MCP
+        和 namespace 等原生专属能力不会静默丢弃：路由会排除 Chat 候选，固定 Chat 链路则返回带参数路径的 unsupported_response_parameter。store
+        缺省为 true。previous_response_id 严格绑定首次成功的平台模型和上游协议，链路不可用时不跨平台续接；未提供 previous_response_id
+        时调用方历史权威，Gateway 不追加本地历史。
       parameters:
-      - description: Responses 请求；Chat 回退只支持自定义 function tools
+      - description: Responses 请求；Chat 回退支持 function/custom tools 及所有可等价能力
         in: body
         name: input
         required: true
@@ -9945,8 +9997,7 @@ paths:
           schema:
             $ref: '#/definitions/httpapi.ResponsesCompatibleResponse'
         "400":
-          description: invalid_previous_response_id / unsupported_response_tool /
-            unsupported_response_parameter
+          description: invalid_previous_response_id / unsupported_response_parameter
           schema:
             $ref: '#/definitions/httpapi.OpenAIErrorEnvelope'
         "401":