[FIX] stock_picking_batch_custom: prevent product_id null error on summary lines

- Use regular dict instead of defaultdict to avoid empty entries
- Make summary_line_ids readonly=True to prevent UI from inserting empty lines
- Add SQL constraint CHECK(product_id IS NOT NULL) as safeguard
- Use boolean_toggle widget for is_collected field
- Fix tests to use TransactionCase and invalidate_recordset
- Add test for empty batch + add pickings + confirm flow

stock_picking_batch_custom/tests/__init__.py

@@ -0,0 +1 @@
+from . import test_batch_summary  # noqa: F401