Issue #3606380: Do not allow flood events when the count query fails

DatabaseBackend::isAllowed() now rethrows the exception when the count query fails but the flood table exists, using the existing catchException() helper. Before, the catch block tried to create the table, swallowed the "table exists" error and returned TRUE, so the flood limit was skipped. A missing table is still created and the event allowed, as before. Adds a unit test for both cases.

Closes #3606380

AI-Generated: Yes (I used Claude to help write the code and tests. I reviewed and tested it before pushing.)