[unstable_after] always use 'waitUntil' from '@next/request-context' if available (#66119)
This PR changes the `getWaitUntil` logic to always check if the platform
provided a request context with a `waitUntil` implementation (via [the
`@next/request-context`
symbol](https://github.com/vercel/next.js/blob/05e6b825768355372dcdfff06dfcd264f78507d3/packages/next/src/server/after/wait-until-builtin.ts#L12))
, regardless of minimalMode or runtime.
Previously, we wouldn't check the context at all unless `minimalMode`
was set, and defaulted `waitUntil` to a noop otherwise. This would make
integration more difficult for providers who don't use `minimalMode`,
because they'd have no way to inject their `waitUntil` implementation.