fix(python): return the session supplied to LanceDBConnection (#4433)

LanceDBConnection.session read AsyncConnection.session, but
AsyncConnection has no session attribute, so access raised
AttributeError. Store the session passed to the constructor and return
that object.

Assisted by Cursor (Grok).

Co-authored-by: Preston Zhao <195942578+canyang25@users.noreply.github.com>

python/python/tests/test_session.py

@@ -36,3 +36,13 @@ def test_session_cache_configuration(tmp_path):
     assert final_cache_size > initial_cache_size  # Cache should have grown
     assert final_cache_items >= initial_cache_items  # Items should not decrease
     assert initial_cache_size < index_cache_size + metadata_cache_size
+
+
+def test_connection_session_returns_supplied_session(tmp_path):
+    """Sync connections must expose the session they were opened with."""
+    db = lancedb.connect(tmp_path)
+    assert db.session is None
+
+    session = lancedb.Session()
+    db = lancedb.connect(tmp_path, session=session)
+    assert db.session is session