fix(conversations): prevent partial updates for invalid assignees (#89719)

Co-authored-by: posthog[bot] <206114724+posthog[bot]@users.noreply.github.com>
Co-authored-by: Paul D'Ambra <paul@posthog.com>
Co-authored-by: tests-posthog[bot] <250237707+tests-posthog[bot]@users.noreply.github.com>
Co-authored-by: pauldambra <984817+pauldambra@users.noreply.github.com>