fix: make database saver classes inheritance-friendly (#2615)

Replace hardcoded database saver class names with `cls` in
`from_conn_string` factory methods to improve subclassing support

## Changes
* Replaced direct class instantiations with `cls(conn)` in
`from_conn_string` classmethods across all database implementations
* Updated both synchronous and asynchronous variants for DuckDB,
PostgreSQL, and SQLite savers

## Why
This refactor makes the database saver classes more extensible by
following Python's convention of using `cls` in class methods. This
enables proper inheritance patterns where subclasses can reuse the
factory methods without needing to override them. Previously, the
hardcoded class names would always instantiate the parent class, even
when called from a subclass.

## Testing
The change is backward compatible and doesn't alter existing
functionality. All existing tests should continue to pass as this is
purely a structural refactoring that preserves the current behavior
while improving extensibility.

## Notes
This PR addresses follow up on comments from #2518 - AsyncPostgresSaver
didn't need to be fixed but many of the other DB saver classes did.

libs/checkpoint-sqlite/langgraph/checkpoint/sqlite/__init__.py

@@ -110,7 +110,7 @@ class SqliteSaver(BaseCheckpointSaver[str]):
                 check_same_thread=False,
             )
         ) as conn:
-            yield SqliteSaver(conn)
+            yield cls(conn)
 
     def setup(self) -> None:
         """Set up the checkpoint database.

libs/checkpoint-sqlite/langgraph/checkpoint/sqlite/aio.py

@@ -137,7 +137,7 @@ class AsyncSqliteSaver(BaseCheckpointSaver[str]):
             AsyncSqliteSaver: A new AsyncSqliteSaver instance.
         """
         async with aiosqlite.connect(conn_string) as conn:
-            yield AsyncSqliteSaver(conn)
+            yield cls(conn)
 
     def get_tuple(self, config: RunnableConfig) -> Optional[CheckpointTuple]:
         """Get a checkpoint tuple from the database.