chore: remove dead table-service transaction machinery

[sshaplygin]

Part of #518.

Context

#518 asks to remove the old transaction API once the query service can span tables and topics (#340). Checking the current state before deleting anything:

• feat: query service: transactions with tables and topics #340 is closed (2026-07-07), so the stated blocker is gone.
• The public side of the removal already happened in the 0.16.0 table/query split. TableClient no longer runs YQL or transactions, there are no #[deprecated] transaction items anywhere, and retry_transaction is already retry_tx.
• Cross-service transactions work today. Topic transactions are built on the query-service Transaction via QueryTxHook — TopicWriterTx::new(.., tx: &mut Transaction), reader.tx_reader(&mut tx), reader.pop_batch_in_tx(&mut tx). All seven integration tests pass against a live server: writer commit/rollback, reader commit/rollback/redelivery/offset-advance, and read_topic_message_in_transaction.

So the API #518 targeted is gone. What remained is the raw plumbing underneath it, which no longer has a caller anywhere in the repository.

What this removes

Removed	Why
execute_data_query.rs, transaction_control.rs, commit_transaction.rs, rollback_transaction.rs	No callers outside their own module
RawTableClient::{execute_data_query, commit_transaction, rollback_transaction}	The only referents of the above
query_stats.rs	RawQueryStatMode existed solely to configure ExecuteDataQuery
Operation impls for ExecuteDataQueryResponse, CommitTransactionResponse, RollbackTransactionResponse	Orphaned with the methods

323 lines deleted, 4 added.

Two things that look similar but are not touched: the commit_transaction / rollback_transaction calls in client_query/exec.rs are the query service's own in raw_query_service, and the surviving RawTxMode is the query-service type, distinct from the table one removed here.

No public API changes. Every removed item was pub(crate) inside a pub(crate) module and never re-exported.

Verification

Against ydbplatform/local-ydb:nightly, the image CI uses:

cargo test --workspace -- --include-ignored   # 302 passed, 0 failed — same count as master
cargo fmt --check
cargo clippy --workspace --all-targets --no-deps --exclude=ydb-grpc -- -D warnings

Note for reviewers

Two follow-ups I deliberately left alone, since neither is transaction-related and both belong in their own change:

• RawTableClient::explain_data_query is also uncalled, as is explain_data_query.rs.
• docker-compose.yaml pins local-ydb:latest, while CI uses :nightly. On :latest, 11 pre-existing integration tests fail with Unknown name: $val.

Whether #518 can now be closed is a maintainer call — this finishes the mechanical part, but the issue was written against a broader API that the 0.16.0 refactor already removed.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.39%. Comparing base (a6d7911) to head (375ff38).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #595      +/-   ##
==========================================
+ Coverage   86.91%   87.39%   +0.48%     
==========================================
  Files         198      193       -5     
  Lines       19492    19373     -119     
==========================================
- Hits        16941    16931      -10     
+ Misses       2551     2442     -109     

Flag	Coverage Δ
rust-1.88.0	87.39% <ø> (+0.48%)	⬆️
rust-1.96.1	87.68% <ø> (+0.53%)	⬆️
tests	87.39% <ø> (+0.48%)	⬆️
ubuntu	87.39% <ø> (+0.48%)	⬆️

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