Fix SingleStoreDB (#10534)

After the refactoring #6570, the DistanceStrategy class was moved to
another module and this introduced a bug into the SingleStoreDB vector
store, as the `DistanceStrategy.EUCLEDIAN_DISTANCE` started to convert
into the 'DistanceStrategy.EUCLEDIAN_DISTANCE' string, instead of just
'EUCLEDIAN_DISTANCE' (same for 'DOT_PRODUCT').

In this change, I check the type of the parameter and use `.name`
attribute to get the correct object's name.

---------

Co-authored-by: Volodymyr Tkachuk <vtkachuk-ua@singlestore.com>

libs/langchain/langchain/vectorstores/singlestoredb.py

@@ -374,7 +374,9 @@ class SingleStoreDB(VectorStore):
                     FROM {} {} ORDER BY __score {} LIMIT %s""".format(
                         self.content_field,
                         self.metadata_field,
-                        self.distance_strategy,
+                        self.distance_strategy.name
+                        if isinstance(self.distance_strategy, DistanceStrategy)
+                        else self.distance_strategy,
                         self.vector_field,
                         self.table_name,
                         where_clause,