🧪 add error testing for batch()

[undivisible]

🎯 What: Adds a missing unit test for the batch() function error handling inside @tschk/moonshine core.
📊 Coverage: Specifically tests that when fn() inside batch(fn) throws an error, the batchDepth is correctly decremented in the finally block, and pending states are still properly restored.
✨ Result: Increased codebase coverage and assurance that signal notifications don't get permanently broken when an unexpected error occurs during a batch update.

---

PR created automatically by Jules for task 14964118668717243256 started by @undivisible

---

Note

Low Risk
Test-only change with no production code modifications.

Overview
Adds a unit test batch decrements depth on error in signal.test.ts to lock in behavior when the callback passed to batch() throws.

The test asserts the error is rethrown, updates applied before the throw still coalesce into one subscriber notification, and a subsequent set outside the failed batch notifies normally—guarding against a stuck batchDepth after exceptions.

^{Reviewed by Cursor Bugbot for commit cb5e017. Configure here.}

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.

[cursor]

Bugbot couldn't run - usage limit reached

Bugbot is counted against Cursor usage for this user or team, and this run hit a usage or spend limit.

A user or team admin can review and increase usage limits in the Cursor dashboard.

(requestId: serverGenReqId_907330ea-1859-457b-ade9-44fd03a92e1b)