sbomexport: make deleted namespace cleanup test wait for handler #510

Description

TestExporterKeepsTheLayoutOfADeletedNamespace waits until the namespace lister no longer contains team-a, then advances the fake clock through the grace period and asserts the namespace metadata was cleaned up. The informer removes an object from the store before asynchronously delivering DeleteFunc; if that handler is delayed until after the resyncs, deletedNamespaces["team-a"] is still unset and the test can fail even though production behavior is correct.

Expected

Before advancing the clock for the cleanup assertion, wait with bounded polling until deletedNamespaces["team-a"] is set under te.mu. Keep TestExporterKeepsTheLayoutOfANamespaceTheListerAlreadyDropped as coverage for the pre-delete-handler interval.

Found during review of #501. This is a test reliability follow-up, not a blocker to the production fix.