Add tests

packages/svelte/tests/runtime-runes/samples/error-boundary-reset-onerror/_config.js

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+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	test({ assert, target, warnings }) {
+		const btn = target.querySelector('button');
+
+		btn?.click();
+
+		assert.throws(() => {
+			flushSync();
+		}, 'error on template render');
+
+		// Check that the warning is being showed to the user
+		assert.include(warnings[0], 'reset() was invoked');
+
+		// boundary content empty; only button remains
+		assert.htmlEqual(target.innerHTML, `<button>trigger throw</button>`);
+	}
+});

packages/svelte/tests/runtime-runes/samples/error-boundary-reset-onerror/main.svelte

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+<script>
+	let must_throw = $state(false);
+
+	function throw_error() {
+		throw new Error("error on template render");
+	}
+</script>
+
+<svelte:boundary onerror={(_, reset) => reset()}>
+	{must_throw ? throw_error() : 'normal content'}
+
+	{#snippet failed()}
+		<div>err</div>
+	{/snippet}
+</svelte:boundary>
+
+<button onclick={() => must_throw = true}>trigger throw</button>

packages/svelte/tests/runtime-runes/samples/error-boundary-reset-premature/_config.js

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+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	html: `
+		normal content
+		<button>toggle</button>
+	`,
+
+	expect_unhandled_rejections: true,
+
+	async test({ assert, target, warnings, window }) {
+		// @ts-expect-error
+		const __expected_error = (window.__expected_error = { v: false });
+
+		window.addEventListener('error', (e) => {
+			// @ts-expect-error when in hydrate mode we can't access variables in the scope
+			const __expected_error = window.__expected_error;
+
+			if (__expected_error.v) {
+				assert.include(e.error.message, 'error on template render');
+			} else {
+				assert.fail('Error was not expected: ' + e.error.message);
+			}
+			e.preventDefault();
+		});
+
+		const btn = target.querySelector('button');
+
+		// 1st click — error caught, fallback visible
+		btn?.click();
+		flushSync();
+		assert.htmlEqual(target.innerHTML, `<div>err</div><button>toggle</button>`);
+
+		// 2nd click — reset succeeds, normal render
+		btn?.click();
+		flushSync();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				normal content
+				<button>toggle</button>
+			`
+		);
+
+		// 3rd click — mount-time crash escapes, boundary empty
+		__expected_error.v = true;
+		btn?.click();
+		flushSync();
+		__expected_error.v = false;
+
+		// Check that the warning is being showed to the user
+		assert.include(warnings[0], 'reset() was invoked');
+
+		// boundary content empty; only button remains
+		assert.htmlEqual(target.innerHTML, `<button>toggle</button>`);
+	}
+});

packages/svelte/tests/runtime-runes/samples/error-boundary-reset-premature/main.svelte

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+<script>
+	let must_throw = $state(false);
+	let reset = $state(null);
+
+	function throw_error() {
+		throw new Error("error on template render");
+	}
+</script>
+
+<svelte:boundary onerror={console.error}>
+	<svelte:boundary onerror={(_, fn) => (reset = fn)}>
+		{must_throw ? throw_error() : 'normal content'}
+
+		{#snippet failed()}
+			<div>err</div>
+		{/snippet}
+	</svelte:boundary>
+</svelte:boundary>
+
+<button
+	onclick={() => {
+		must_throw = !must_throw;
+		if (reset) reset();
+	}}>
+	toggle
+</button>