fix: allow deleting non-existent `$$restProps` properties (#12971)

fixes #12966

.changeset/light-tigers-smoke.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: allow deleting non-existent `$$restProps` properties

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

@@ -130,7 +130,7 @@ const legacy_rest_props_handler = {
 	},
 	deleteProperty(target, key) {
 		// Svelte 4 allowed for deletions on $$restProps
-		if (target.exclude.includes(key)) return false;
+		if (target.exclude.includes(key)) return true;
 		target.exclude.push(key);
 		update(target.version);
 		return true;

packages/svelte/tests/runtime-legacy/samples/rest-props-delete/App.svelte

@@ -2,6 +2,8 @@
 	function prune() {
 		$$restProps.a
 		delete $$restProps.a
+		// should be idempotent
+		delete $$restProps.a
 	}
 </script>