fix: handle undefined sorts/customColumns from lazy+StrictMode defaultProps bug

[mofojed]

When IrisGrid is loaded via LazyIrisGrid (the public export), it goes
through a forwardRef -> lazy() -> Suspense chain. In React 18 StrictMode,
there is a known bug (facebook/react#28505) where defaultProps are not
correctly merged into this.props during the second componentDidMount
call of the StrictMode double-mount cycle.

The root cause is in React's reconciler: during the simulated unmount,
componentWillUnmount sets instance.props to fiber.memoizedProps (the
unresolved props without defaultProps merged). When React re-mounts the
component, componentDidMount sees these unresolved props, so props like
sorts and customColumns are undefined instead of their defaultProps
values (EMPTY_ARRAY).

loadTableState() then sets these undefined values into state, and
IrisGridModelUpdater crashes with 'sorts is not iterable' when it
tries to spread them: [...sorts].

Add defensive ?? EMPTY_ARRAY fallbacks in:

• IrisGrid.loadTableState(): when setting sorts/customColumns into state
• IrisGridModelUpdater: when spreading sorts and assigning customColumns

Add tests that reproduce the exact bug by combining async lazy loading
with StrictMode, and unit tests for IrisGridModelUpdater with undefined
sorts/customColumns.

[mofojed]

The bug is a React issue: facebook/react#28505
It only manifests in development mode when using StrictMode. I think I'd rather have it fail immediately than fail in other subtle ways. It does not affect production.
The correct fix is to convert to functional components and/or update to React v19. I'm going to close this ticket so it still fails hard and can be found when searching for issues, pointing to the React issue.