chore: robustify `flatten` (#17864)

Extracted from #17805. Currently we restore context in`flatten`
unnecessarily in the case where we have async expressions but no
blockers (the context is already correct), and we don't unset context
after blockers resolve in the case where we have them. The first bit is
suboptimal, but the second bit feels bug-shaped, even though I'm not
currently aware of any actual bugs that have resulted from this.

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packages/svelte/src/internal/client/reactivity/async.js

@@ -76,14 +76,17 @@ export function flatten(blockers, sync, async, fn) {
 
 	// Full path: has async expressions
 	function run() {
-		restore();
 		Promise.all(async.map((expression) => async_derived(expression)))
 			.then((result) => finish([...sync.map(d), ...result]))
 			.catch((error) => invoke_error_boundary(error, parent));
 	}
 
 	if (blocker_promise) {
-		blocker_promise.then(run);
+		blocker_promise.then(() => {
+			restore();
+			run();
+			unset_context();
+		});
 	} else {
 		run();
 	}