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/httpapi/responses_streaming_test.go

@@ -34,3 +34,16 @@ func TestResponsesStreamWriterForwardsStandardEventsAndNeverWritesDoneMarker(t *
 		t.Fatalf("Responses stream duplicated response.completed: %s", body)
 	}
 }
+
+func TestResponsesStreamWriterUsesIncompleteTerminalWithoutAppendingCompleted(t *testing.T) {
+	recorder := httptest.NewRecorder()
+	writer := newCompatibleStreamWriter("responses", "demo", false)
+	writer.writeDelta(recorder, clients.StreamDeltaEvent{Event: map[string]any{
+		"type": "response.incomplete", "sequence_number": 3, "response": map[string]any{"id": "resp_123", "status": "incomplete"},
+	}})
+	writer.writeDone(recorder, map[string]any{"id": "resp_123", "status": "incomplete"})
+	body := recorder.Body.String()
+	if strings.Count(body, "event: response.incomplete") != 1 || strings.Contains(body, "event: response.completed") {
+		t.Fatalf("incomplete terminal event was duplicated or rewritten: %s", body)
+	}
+}