fix(graph): allow property names with different cases (#149)

* fix(graph): allow property names with different cases

* Format code

---------

Co-authored-by: Averi Kitsch <akitsch@google.com>

Author: mtyin

src/langchain_google_spanner/graph_store.py

@@ -901,10 +901,12 @@ def __repr__(self) -> str:
         Returns:
           str: a string representation of the graph schema.
         """
-        properties = {
-            k: TypeUtility.spanner_type_to_schema_str(v)
-            for k, v in self.properties.items()
-        }
+        properties = CaseInsensitiveDict(
+            {
+                k: TypeUtility.spanner_type_to_schema_str(v)
+                for k, v in self.properties.items()
+            }
+        )
         return json.dumps(
             {
                 "Name of graph": self.graph_name,