TransactWriteItems leaves index write units out of ConsumedCapacity #866

What is wrong

With ReturnConsumedCapacity set to INDEXES, Amazon DynamoDB reports the units a transaction writes to each global secondary index, and counts them in the total. ExtendDB reports the table alone.

Measured in us-east-1 on 2026-11-05, on a table with one global secondary index, gsi:

ExecuteTransaction on #411 gives the same numbers as TransactWriteItems on both sides, because it runs through the same code.

Where

crates/engine/src/transact_write_items.rs, at the note that the per-index breakdown for transactions is deferred: the engine has no old or new item for an Update or Delete member, so it cannot tell which index entries a write changes. The storage layer's transact_write_items returns neither on any of the four backends.

What to change

1. Have transact_write_items on SQLite, PostgreSQL, MongoDB and Cassandra return the old and the new item of each member, or the index entries each member wrote.
2. Rate each member's index writes as the single-item writes do, and add them to the index entries and to the total.
3. Remove the expected-failure marks on test_a_transaction_reports_its_index_units and test_a_transaction_total_counts_its_index_units in tests/test_partiql_transaction.py (from feat(partiql): implement ExecuteStatement, BatchExecuteStatement, ExecuteTransaction #411), and add the same cases for TransactWriteItems.