fix: replace db.fetch with db.query for PostgreSQL migration

Why this change is needed:
PostgreSQLDB class doesn't have a fetch() method. The migration code
was incorrectly using db.fetch() for batch data retrieval, causing
AttributeError during E2E tests.

How it solves it:
1. Changed db.fetch(sql, params) to db.query(sql, params, multirows=True)
2. Updated all test mocks to support the multirows parameter
3. Consolidated mock_query implementation to handle both single and multi-row queries

Impact:
- PostgreSQL legacy data migration now works correctly in E2E tests
- All unit tests pass (6/6)
- Aligns with PostgreSQLDB's actual API

Testing:
- pytest tests/test_postgres_migration.py -v (6/6 passed)
- Updated test_postgres_migration_trigger mock
- Updated test_scenario_2_legacy_upgrade_migration mock
- Updated base mock_pg_db fixture

lightrag/kg/postgres_impl.py

@@ -2319,7 +2319,7 @@ class PGVectorStorage(BaseVectorStorage):
                 select_query = (
                     f"SELECT * FROM {legacy_table_name} OFFSET $1 LIMIT $2"
                 )
-                rows = await db.fetch(select_query, [offset, batch_size])
+                rows = await db.query(select_query, [offset, batch_size], multirows=True)
 
                 if not rows:
                     break

tests/test_postgres_migration.py

@@ -19,10 +19,15 @@ def mock_pg_db():
     db = AsyncMock()
     db.workspace = "test_workspace"
 
-    # Mock query responses
-    db.query = AsyncMock(return_value={"exists": False, "count": 0})
+    # Mock query responses with multirows support
+    async def mock_query(sql, params=None, multirows=False, **kwargs):
+        # Default return value
+        if multirows:
+            return []  # Return empty list for multirows
+        return {"exists": False, "count": 0}
+
+    db.query = AsyncMock(side_effect=mock_query)
     db.execute = AsyncMock()
-    db.fetch = AsyncMock(return_value=[])
 
     return db
 
@@ -108,26 +113,24 @@ async def test_postgres_migration_trigger(mock_client_manager, mock_pg_db, mock_
         return table_name == storage.legacy_table_name
 
     # 2. Legacy table has 100 records
-    async def mock_query(sql, params):
-        if "COUNT(*)" in sql:
-            return {"count": 100}
-        return {}
-
-    # 3. Mock fetch for batch migration
     mock_rows = [
         {"id": f"test_id_{i}", "content": f"content_{i}", "workspace": "test_ws"}
         for i in range(100)
     ]
 
-    async def mock_fetch(sql, params):
-        offset = params[0] if params else 0
-        limit = params[1] if len(params) > 1 else 500
-        start = offset
-        end = min(offset + limit, len(mock_rows))
-        return mock_rows[start:end]
+    async def mock_query(sql, params=None, multirows=False, **kwargs):
+        if "COUNT(*)" in sql:
+            return {"count": 100}
+        elif multirows and "SELECT *" in sql:
+            # Mock batch fetch for migration
+            offset = params[0] if params else 0
+            limit = params[1] if len(params) > 1 else 500
+            start = offset
+            end = min(offset + limit, len(mock_rows))
+            return mock_rows[start:end]
+        return {}
 
     mock_pg_db.query = AsyncMock(side_effect=mock_query)
-    mock_pg_db.fetch = AsyncMock(side_effect=mock_fetch)
 
     with patch("lightrag.kg.postgres_impl._pg_table_exists", side_effect=mock_table_exists), \
          patch("lightrag.kg.postgres_impl._pg_create_table", AsyncMock()):
@@ -253,30 +256,28 @@ async def test_scenario_2_legacy_upgrade_migration(mock_client_manager, mock_pg_
         return table_name == storage.legacy_table_name
 
     # Mock: legacy table has 50 records
-    async def mock_query(sql, params):
+    mock_rows = [
+        {"id": f"legacy_id_{i}", "content": f"legacy_content_{i}", "workspace": "legacy_workspace"}
+        for i in range(50)
+    ]
+
+    async def mock_query(sql, params=None, multirows=False, **kwargs):
         if "COUNT(*)" in sql:
             # First call for legacy count, then for verification
             if storage.legacy_table_name in sql:
                 return {"count": 50}
             else:
                 return {"count": 50}
+        elif multirows and "SELECT *" in sql:
+            # Mock batch fetch for migration
+            offset = params[0] if params else 0
+            limit = params[1] if len(params) > 1 else 500
+            start = offset
+            end = min(offset + limit, len(mock_rows))
+            return mock_rows[start:end]
         return {}
 
-    # Mock fetch for migration
-    mock_rows = [
-        {"id": f"legacy_id_{i}", "content": f"legacy_content_{i}", "workspace": "legacy_workspace"}
-        for i in range(50)
-    ]
-
-    async def mock_fetch(sql, params):
-        offset = params[0] if params else 0
-        limit = params[1] if len(params) > 1 else 500
-        start = offset
-        end = min(offset + limit, len(mock_rows))
-        return mock_rows[start:end]
-
     mock_pg_db.query = AsyncMock(side_effect=mock_query)
-    mock_pg_db.fetch = AsyncMock(side_effect=mock_fetch)
 
     with patch("lightrag.kg.postgres_impl._pg_table_exists", side_effect=mock_table_exists), \
          patch("lightrag.kg.postgres_impl._pg_create_table", AsyncMock()) as mock_create: