refactor: iterate migration result cursor directly

Addresses Copilot review round 2 on #2358; matches the cb8df44ebda5 style.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TaMepbxRYzJxQKFtu6Hq4p
Signed-off-by: F.N. Claessen <felix@seita.nl>

Author: Flix6x

flexmeasures/data/migrations/versions/3c2f9e5a1d47_split_inflexible_device_sensors_by_sign.py

@@ -138,7 +138,7 @@ def _rewrite_flex_contexts(transform) -> None:
     conn = op.get_bind()
     for asset_id, flex_context in conn.execute(
         sa.select(generic_asset_table.c.id, generic_asset_table.c.flex_context)
-    ).fetchall():
+    ):
         if not flex_context:
             continue
         changed = transform(conn, flex_context)