Throw on ambiguous 404s instead of returning `undefined`

[vdusek]

Every 404 is swallowed into undefined. Where a 404 can mean either the parent or the sub-resource is missing, that hides a real error: run.dataset() / keyValueStore() / requestQueue() / log(), dataset.getStatistics(), user.monthlyUsage() and limits(), schedule.getLog(), task.getInput(), webhook.test(). A direct get(id) / delete(id) keeps returning undefined.

Breaking — the affected return types drop | undefined. Needs the error subclasses from #709 first.

Python: apify/apify-client-python#755, on top of apify/apify-client-python#737.

✍️ Drafted by Claude Code

[vdusek]

resolved by #1042