[FIX] core: always close cursors at the exit of context managers

The call to `commit()` can fail during the flush of the environment
and raise an exception before the actual `COMMIT;`, leaving the
cursor unclosed.

This leaked cursor may hold locks that only be released when the GC
collects it. On low-traffic workers like the CronWorker, this can
lock usual database usages.

X-original-commit: ccb58667a475a062e7d27dd39ff9d01ca0bdb68d
Part-of: odoo/odoo#93866