Refuse sqlite3_rekey_v2 while a transaction is open - #627

On 5.0.0-beta, sqlite3_rekey_v2 commits its btree transaction even when the connection already holds one, so it ends a transaction it never owned. Inside BEGIN or SAVEPOINT, a later ROLLBACK then restores nothing. From a callback of a running INSERT, it commits the partial writes, and an INSERT that then fails stays half applied. Alongside a SELECT still stepping, it drops the read lock under the open cursor, which trips an assertion in SQLITE_DEBUG builds.

As PRAGMA cipher_migrate already does, sqlite3_rekey_v2 now returns SQLITE_MISUSE when autocommit is off or the target btree holds any transaction. The check runs before anything is touched and returns directly, since the cleanup path would roll back the caller's transaction.

The tests cover BEGIN, SAVEPOINT, an attached schema, a running INSERT and a running SELECT.