fix: use NetworkXStorage for E2E tests (AGE extension not available in CI)

Why this change is needed:
E2E PostgreSQL tests were failing because they specified graph_storage="PGGraphStorage",
but the CI environment doesn't have the Apache AGE extension installed. This caused
initialize_storages() to fail with "function create_graph(unknown) does not exist".

How it solves it:
Removed graph_storage="PGGraphStorage" parameter in all PostgreSQL E2E tests,
allowing LightRAG to use the default NetworkXStorage which doesn't require
external dependencies.

Impact:
- PostgreSQL E2E tests can now run successfully in CI
- Vector storage migration tests can complete without AGE extension dependency
- Maintains test coverage for vector storage model isolation feature

Testing:
The vector storage migration tests (which are the focus of this PR) don't
depend on graph storage implementation and can run with NetworkXStorage.

tests/test_e2e_multi_instance.py

@@ -259,7 +259,7 @@ async def test_legacy_migration_postgres(
             tokenizer=mock_tokenizer,
             kv_storage="PGKVStorage",
             vector_storage="PGVectorStorage",
-            graph_storage="PGGraphStorage",
+            # Use default NetworkXStorage for graph storage (AGE extension not available in CI)
             doc_status_storage="PGDocStatusStorage",
             vector_db_storage_cls_kwargs={
                 **pg_config,
@@ -487,7 +487,7 @@ async def test_multi_instance_postgres(
         tokenizer=mock_tokenizer,
         kv_storage="PGKVStorage",
         vector_storage="PGVectorStorage",
-        graph_storage="PGGraphStorage",
+        # Use default NetworkXStorage for graph storage (AGE extension not available in CI)
         doc_status_storage="PGDocStatusStorage",
         vector_db_storage_cls_kwargs={
             **pg_config,
@@ -508,7 +508,7 @@ async def test_multi_instance_postgres(
         tokenizer=mock_tokenizer,
         kv_storage="PGKVStorage",
         vector_storage="PGVectorStorage",
-        graph_storage="PGGraphStorage",
+        # Use default NetworkXStorage for graph storage (AGE extension not available in CI)
         doc_status_storage="PGDocStatusStorage",
         vector_db_storage_cls_kwargs={
             **pg_config,