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/internal/runner/service.go

@@ -481,7 +481,7 @@ func (s *Service) executeWithToken(ctx context.Context, task store.GatewayTask,
 		return Result{Task: failed, Output: failed.Result}, err
 	}
 	if task.Kind == "responses" {
-		candidates, err = prepareResponseCandidates(candidates, responseExecution)
+		candidates, err = prepareResponseCandidates(candidates, responseExecution, body)
 		if err != nil {
 			code, message := responseExecutionFailure(err)
 			s.recordFailedAttempt(ctx, failedAttemptRecord{Task: task, Body: body, AttemptNo: task.AttemptCount + 1, Code: code, Cause: err, Simulated: task.RunMode == "simulation", Scope: "response_protocol", Reason: code, ModelType: modelType})