fix(sql-workspace): cancelling the destructive-query dialog still throws away the staged table edits #1771

Problem

In `generic_sql_workspace.dart` (run path), `confirmDiscardTableEditsIfDirty` is asked first, and when the user agrees the staging buffer is disposed immediately. The destructive-query confirmation comes after that. If the user then cancels it, the query does not run, but the staged edits are already gone.

Fix

Dispose the buffer only after every confirmation has passed (just before the query starts), or ask the destructive question first. Add a widget test: dirty buffer, destructive SQL, answer "discard" and then "cancel" keeps the buffer.

Found in the review of the SQL safety layer.