test: 更新 MeSH 打标测试适配 tag_service 重构

- _tag_article → tag_service.tag_article 导入路径
- 增加 name_en 查询 mock 返回值（3 次 execute 调用）

backend/tests/test_service_pubmed_api.py

@@ -155,9 +155,9 @@ async def test_tag_article_no_mesh():
     """Empty mesh_headings -> returns 0"""
     db = AsyncMock()
 
-    from app.services.pubmed_api import _tag_article
+    from app.services.tag_service import tag_article
 
-    result = await _tag_article(db, uuid.uuid4(), [])
+    result = await tag_article(db, uuid.uuid4(), [])
     assert result == 0
 
 
@@ -166,9 +166,9 @@ async def test_tag_article_no_mesh_ui():
     """Mesh headings without UI values -> returns 0"""
     db = AsyncMock()
 
-    from app.services.pubmed_api import _tag_article
+    from app.services.tag_service import tag_article
 
-    result = await _tag_article(
+    result = await tag_article(
         db, uuid.uuid4(),
         [{"descriptor": "Cancer", "ui": "", "major": False}],
     )
@@ -188,15 +188,17 @@ async def test_tag_article_matches_tags():
 
     mock_match = MagicMock()
     mock_match.scalars.return_value.all.return_value = [mock_tag]
-    mock_empty = MagicMock()
-    mock_empty.scalar.return_value = None
-    db.execute.side_effect = [mock_match, mock_empty]
+    mock_no_name = MagicMock()
+    mock_no_name.scalars.return_value.all.return_value = []
+    mock_no_existing = MagicMock()
+    mock_no_existing.all.return_value = []
+    db.execute.side_effect = [mock_match, mock_no_name, mock_no_existing]
 
     mesh_headings = [{"descriptor": "Cancer", "ui": "D000001", "major": True}]
 
-    from app.services.pubmed_api import _tag_article
+    from app.services.tag_service import tag_article
 
-    result = await _tag_article(db, lit_id, mesh_headings)
+    result = await tag_article(db, lit_id, mesh_headings)
     assert result == 1
     db.add.assert_called_once()
 
@@ -213,9 +215,9 @@ async def test_tag_article_no_match():
 
     mesh_headings = [{"descriptor": "RareDisease", "ui": "D999999", "major": False}]
 
-    from app.services.pubmed_api import _tag_article
+    from app.services.tag_service import tag_article
 
-    result = await _tag_article(db, lit_id, mesh_headings)
+    result = await tag_article(db, lit_id, mesh_headings)
     assert result == 0
     db.add.assert_not_called()