fix(checkpoint-postgres): ensure vector extension is created only if not exists (#6154)

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- **Description:** Azure Postgres SQL server has a limitation when doing
create extension vector is not exists, even though it manually created
before on a schema.
- **Issue:** Even though `CREATE EXTENSION vector` is executed manually
before, the permission issue arises. Putting it in an if else block
solves the issue and its not a breaking change.
```
Because vector isn't a trusted extension, only members of "azure_pg_admin" are allowed to use CREATE EXTENSION vector
HINT: to learn how to allow an extension or see the list of allowed extensions, please refer to https://go.microsoft.com/fwlink/?linkid=2301063
```

Co-authored-by: Josh Rogers <josh@langchain.dev>

libs/checkpoint-postgres/langgraph/store/postgres/base.py

@@ -91,7 +91,12 @@ WHERE expires_at IS NOT NULL;
 VECTOR_MIGRATIONS: Sequence[Migration] = [
     Migration(
         """
-CREATE EXTENSION IF NOT EXISTS vector;
+DO $$
+BEGIN
+    IF NOT EXISTS (SELECT 1 FROM pg_extension WHERE extname = 'vector') THEN
+        CREATE EXTENSION vector;
+    END IF;
+END $$;
 """,
     ),
     Migration(