feat(go-models): migrate batch 5 model drivers to unified handlers (#17700)

## Summary

Relate to #17284. Completes the batch 5 migration of 7 OpenAI-compatible
drivers (`vllm`, `volcengine`, `xai`, `xiaomi`, `xinference`, `xunfei`,
`zhipu-ai`) onto the unified request/response helpers
(`doRequest`/`doStreamRequest` +
`HandleNonStreamingResponse`/`HandleStreamingResponse` +
`ParserConfig`), established by `deepseek` in #17634.

This branch is rebased on the current `pr/migrate-models-batch5` and
fixes the issues in the previous state of the PR.

Co-authored-by: Haruko386 <tryeverypossible@163.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/entity/models/response_handler_test.go

@@ -0,0 +1,81 @@
+//
+//  Copyright 2026 The InfiniFlow Authors. All Rights Reserved.
+//
+//  Licensed under the Apache License, Version 2.0 (the "License");
+//  you may not use this file except in compliance with the License.
+//  You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+//  Unless required by applicable law or agreed to in writing, software
+//  distributed under the License is distributed on an "AS IS" BASIS,
+//  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+//  See the License for the specific language governing permissions and
+//  limitations under the License.
+//
+
+package models
+
+import (
+	"bytes"
+	"io"
+	"strings"
+	"testing"
+)
+
+// TestHandleStreamingResponseBareRootFinishReason verifies the shared handler
+// terminates the stream when an event carries only a root-level finish_reason
+// with no choices and no trailing [DONE]. Before the fix this event was
+// skipped by the choices guard and the handler returned "stream ended before
+// [DONE] or finish_reason".
+func TestHandleStreamingResponseBareRootFinishReason(t *testing.T) {
+	sse := "data: {\"finish_reason\":\"stop\"}\n"
+
+	var contentChunks, reasonChunks []string
+	err := HandleStreamingResponse(
+		io.NopCloser(bytes.NewBufferString(sse)),
+		nil,
+		nil,
+		OpenAIParserConfig,
+		func(content *string, reason *string) error {
+			if content != nil && *content != "" {
+				contentChunks = append(contentChunks, *content)
+			}
+			if reason != nil && *reason != "" {
+				reasonChunks = append(reasonChunks, *reason)
+			}
+			return nil
+		},
+	)
+	if err != nil {
+		t.Fatalf("HandleStreamingResponse: %v", err)
+	}
+
+	// The terminal [DONE] marker is the only content the handler emits for a
+	// finish_reason-only event: no tokens arrived and no reasoning was sent.
+	if got := strings.Join(contentChunks, ""); got != "[DONE]" {
+		t.Errorf("content=%q, want only the [DONE] marker", got)
+	}
+	if len(reasonChunks) != 0 {
+		t.Errorf("reasoning=%q, want none for a finish_reason-only event", strings.Join(reasonChunks, ""))
+	}
+}
+
+// TestHandleStreamingResponseTruncatedStream verifies a stream that ends
+// without [DONE], a root-level finish_reason, or a per-choice finish_reason
+// is rejected as truncated. This is the counterpart of the bare
+// finish_reason case: only a genuine truncation must fail.
+func TestHandleStreamingResponseTruncatedStream(t *testing.T) {
+	sse := "data: {\"choices\":[{\"delta\":{\"content\":\"partial\"}}]}\n"
+
+	err := HandleStreamingResponse(
+		io.NopCloser(bytes.NewBufferString(sse)),
+		nil,
+		nil,
+		OpenAIParserConfig,
+		func(*string, *string) error { return nil },
+	)
+	if err == nil || !strings.Contains(err.Error(), "stream ended before [DONE] or finish_reason") {
+		t.Fatalf("expected truncation error, got %v", err)
+	}
+}