fix: ensure hydration state restoration for custom element attribute updates (#18467)

### Problem
While hydrating custom elements, set_attributes temporarily disables
global hydration state via set_hydrating(false) and restores it only at
the end. If a prop setter or attribute operation throws partway through,
hydration mode can remain disabled globally.

### Fix
Wrap the temporary hydration-state override in a try/finally block in
set_attributes so set_hydrating(true) always runs when
is_hydrating_custom_element is true.

---------

Co-authored-by: Simon Holthausen <simon.holthausen@vercel.com>

.changeset/hydration-custom-element-error-boundary.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: restore hydration state when custom element attribute updates throw

packages/svelte/src/internal/client/dom/elements/attributes.js

@@ -298,6 +298,7 @@ function set_attributes(
 	var is_option_element = element.nodeName === OPTION_TAG;
 	var is_select_element = element.nodeName === SELECT_TAG;
 
+	try {
 		for (var key in prev) {
 			// don't null our internal $$onX listeners
 			if (!(key in next) && key[0] + key[1] !== '$$') {
@@ -317,7 +318,11 @@ function set_attributes(
 
 		var setters = get_setters(element);
 
-	if (element.nodeName === INPUT_TAG && 'type' in next && ('value' in next || '__value' in next)) {
+		if (
+			element.nodeName === INPUT_TAG &&
+			'type' in next &&
+			('value' in next || '__value' in next)
+		) {
 			var type = next.type;
 
 			if (type !== current.type || (type === undefined && element.hasAttribute('type'))) {
@@ -485,10 +490,11 @@ function set_attributes(
 				}
 			}
 		}
-
+	} finally {
 		if (is_hydrating_custom_element) {
 			set_hydrating(true);
 		}
+	}
 
 	return current;
 }

packages/svelte/tests/runtime-runes/samples/hydration-custom-element-error-boundary/_config.js

@@ -0,0 +1,11 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	mode: ['hydrate'],
+
+	async test({ assert, target }) {
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<p>failed: setter error</p><p>after</p>');
+	}
+});

packages/svelte/tests/runtime-runes/samples/hydration-custom-element-error-boundary/main.svelte

@@ -0,0 +1,22 @@
+<script module>
+	if (!customElements.get('throwing-element')) {
+		customElements.define(
+			'throwing-element',
+			class extends HTMLElement {
+				set value(_) {
+					throw new Error('setter error');
+				}
+			}
+		);
+	}
+</script>
+
+<svelte:boundary>
+	<throwing-element {...{ value: 'boom' }}></throwing-element>
+
+	{#snippet failed(error)}
+		<p>failed: {error.message}</p>
+	{/snippet}
+</svelte:boundary>
+
+<p>after</p>