fix: update_version after delete if source is undefined and prop in target

.changeset/great-colts-film.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: `update_version` after `delete` if `source` is `undefined` and `prop` in `target`

packages/svelte/src/internal/client/proxy.js

@@ -100,6 +100,7 @@ export function proxy(value) {
 						prop,
 						with_parent(() => source(UNINITIALIZED, stack))
 					);
+					update_version(version);
 				}
 			} else {
 				// When working with arrays, we need to also ensure we update the length when removing

packages/svelte/tests/runtime-runes/samples/delete-proxy-key/_config.js

@@ -0,0 +1,13 @@
+import { test } from '../../test';
+import { flushSync } from 'svelte';
+
+export default test({
+	html: `<button>delete</button><p>test</p>`,
+
+	async test({ assert, target }) {
+		const btn = target.querySelector('button');
+
+		flushSync(() => btn?.click());
+		assert.htmlEqual(target.innerHTML, '<button>delete</button>');
+	}
+});

packages/svelte/tests/runtime-runes/samples/delete-proxy-key/main.svelte

@@ -0,0 +1,13 @@
+<script>
+	let obj = $state({test: 0})
+
+	let keys = $derived(Object.keys(obj));
+</script>
+
+<button onclick={() => delete obj.test}>
+	delete
+</button>
+
+{#each keys as key}
+	<p>{key}</p>
+{/each}