fix: disallow effect creation after `await` (#18299)

We incorrectly restore effect context after an`$derived(await ...)` if
it occurs inside an async function, but only for those, no other `await`
expressions. This is buggy and wrong. We only want to restore context
_inside_ an async derived expression (including template expressions)`
so that `await a + b` works correctly.

It's possible that this will be a breaking change (albeit not
semver-violating, because `experimental`) for some people if they are
creating effects after an `await` (other than at the top level of a
component). This is regrettable, but it should never have worked in the
first place.