test(e2e): SQL transaction controls and destructive guard

[ZhuchkaTriplesix]

Part of #1051.

test/e2e/02_sql_workspaces/e2e_transaction_guard_test.dart:

• Begin / Commit / Rollback drive a transaction and the Transaction: open / none label follows the database;
• a database without transactions shows no transaction controls;
• a destructive statement waits for confirmation, Cancel sends nothing;
• closing the workspace while a query runs cancels it.

EXPLAIN is not exercised: the generic workspace has no EXPLAIN control (explainQuery is only implemented by the per-database delegates). The 50 000-row virtual scroll scenario is also still open.