[codex] Fix SQL BEGIN transaction tracking

[bill-ph]

Summary

• track raw SQL BEGIN and START TRANSACTION statements in the shared SQL transaction-state helper
• update the Flight SQL update path to always apply SQL transaction-state tracking after statement execution
• add regression coverage for successful and failed SQL-level transaction starts

Why

A raw SQL BEGIN was intentionally excluded from isTransactionControlStmt, but session.sqlTxActive was only updated inside the transaction-control guard. That meant SQL-level transactions were not marked active after BEGIN, so later statements could be retried as if they were autocommit and mask the original transaction failure.

Validation

• go test ./duckdbservice/...
• just lint