fix: unlink errored and otherwise finished batch (#18264)

This fixes the issue in
https://github.com/sveltejs/svelte/issues/18221#issuecomment-4497918414
where an error can create follup-up invariant errors. The batch errors,
has no chance to run otherwise (no more pending work) and is therefore
"dead". That means we need to unlink it otherwise it's becoming a
"zombie" and hangs around, causing unnecessary and potentially buggy (as
seen in the reproduction) merge/commit work.

I was not able to reduce the reproduction down to a test case that fails
without the fix, but it does make a related error test from #17888 work
more correctly.

.changeset/beige-bobcats-eat.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: unlink errored and otherwise finished batch

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

@@ -326,6 +326,12 @@ export class Batch {
 				this.#traverse(root, effects, render_effects);
 			} catch (e) {
 				reset_all(root);
+				// If there's no async work left, this branch is now dead and needs
+				// to be unlinked to not become a zombie that is never cleaned up.
+				// See https://github.com/sveltejs/svelte/issues/18221#issuecomment-4497918414
+				// for a (non-minimal) reproduction that demonstrates a case where this is necessary
+				// to not get follow-up false-positives via "batch has scheduled roots" invariant errors.
+				if (!this.#is_deferred()) this.#unlink();
 				throw e;
 			}
 		}

packages/svelte/tests/runtime-runes/samples/error-recovery/_config.js

@@ -2,7 +2,7 @@ import { flushSync } from 'svelte';
 import { test } from '../../test';
 
 export default test({
-	async test({ assert, target, compileOptions }) {
+	async test({ assert, target }) {
 		const [toggle, increment] = target.querySelectorAll('button');
 
 		flushSync(() => increment.click());
@@ -25,8 +25,8 @@ export default test({
 			`
 				<button>toggle</button>
 				<button>count: 2</button>
-				<p>show: ${compileOptions.experimental?.async ? 'false' : 'true'}</p>
-			`
+				<p>show: true</p>
+			` // show: false would also be fine; this is more about ensuring that things continue to work _somehow_
 		);
 	}
 });