fix: make Object.hasOwn reactive for state proxies (#18838)

Fixes #18837

### What changed

State proxies now establish a per-property reactive dependency when
`Object.hasOwn(proxy, prop)` invokes the `getOwnPropertyDescriptor`
trap. Property additions and deletions therefore invalidate the
expression, consistently with the `prop in proxy` operator.

The source creation uses the same eligibility constraints as the
existing `has` trap so inherited properties retain their existing
semantics.

A runtime-runes regression sample covers both adding and deleting the
observed property in DOM and hydration modes.

### Tests and linting

- `FILTER=object-has-own-reactive pnpm test runtime-runes
--reporter=dot` — 4 passed
- `pnpm exec vitest run
packages/svelte/src/internal/client/proxy.test.ts --reporter=dot` — 8
passed
- `pnpm test runtime-runes --reporter=dot` — 2707 passed, 34 skipped
- `pnpm test --reporter=dot` — 7786 passed, 55 skipped
- `cd packages/svelte && pnpm check` — passed
- `pnpm format` — passed
- `pnpm lint` — passed

Includes a patch changeset for `svelte`.

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---------

Co-authored-by: svelte-triage-bot <team@svelte.com>
Co-authored-by: Rich Harris <rich.harris@vercel.com>

.changeset/reactive-has-own.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: make Object.hasOwn reactive for state proxy ownership changes

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

@@ -204,16 +204,21 @@ export function proxy(value) {
 		},
 
 		getOwnPropertyDescriptor(target, prop) {
+			this.has?.(target, prop);
+
 			var descriptor = Reflect.getOwnPropertyDescriptor(target, prop);
+			var s = sources.get(prop);
 
-			if (descriptor && 'value' in descriptor) {
-				var s = sources.get(prop);
-				if (s) descriptor.value = get(s);
-			} else if (descriptor === undefined) {
-				var source = sources.get(prop);
-				var value = source?.v;
+			if (s !== undefined) {
+				var value = get(s);
+
+				if (value === UNINITIALIZED) {
+					return undefined;
+				}
 
-				if (source !== undefined && value !== UNINITIALIZED) {
+				if (descriptor && 'value' in descriptor) {
+					descriptor.value = value;
+				} else {
 					return {
 						enumerable: true,
 						configurable: true,

packages/svelte/tests/runtime-runes/samples/object-has-own-reactive/_config.js

@@ -0,0 +1,24 @@
+import { test } from '../../test';
+import { tick } from 'svelte';
+
+export default test({
+	html: `<button>add y</button><button>delete y</button><p>false</p>`,
+
+	async test({ assert, target }) {
+		const [add, remove] = target.querySelectorAll('button');
+
+		add.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`<button>add y</button><button>delete y</button><p>true</p>`
+		);
+
+		remove.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`<button>add y</button><button>delete y</button><p>false</p>`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/object-has-own-reactive/main.svelte

@@ -0,0 +1,7 @@
+<script>
+	let state = $state({});
+</script>
+
+<button onclick={() => (state.y = true)}>add y</button>
+<button onclick={() => delete state.y}>delete y</button>
+<p>{Object.hasOwn(state, 'y')}</p>