Keep chat subscribers alive after a rejected changeApplied callback

[hidcc]

Fixes #305.

What does this change?

Since #275, a rejected changeApplied callback removes the subscriber from #chatSubscribers (and the subscribeToChat replay path calls unsubscribe on rejection), even though one rejected callback does not mean the RPC connection is broken. The page stays connected but permanently unsubscribed, so the chat UI silently stops updating during an agent turn until a refresh re-subscribes it.

This patch makes both changeApplied rejection handlers (the live broadcast in emitChatChangeApplied and the retained-row replay in subscribeToChat) log the rejection and keep the subscription, matching the mitigation described in #305. Dead clients are still removed through the existing subscriber.onRpcBroken(() => unsubscribe()) handler.

Why is this obviously correct and trivially verifiable?

The patch only changes what happens after a changeApplied promise rejects: previously the subscriber was dropped (silently, with the error swallowed); now the error is logged with the existing logger convention and the subscriber is kept. No control flow before the .catch changes, and disconnect cleanup is unchanged (onRpcBroken → unsubscribe, visible a few lines above the replay site).

Verified locally: a regression test (a subscriber whose changeApplied rejects once must still receive the next row) fails on main and passes with this patch — kept out of this PR to stay within the line limit, available at hidcc:regression-test-305. The full chat-changes.test.ts suite passes, and vp lint / tsc -p packages/workshop-backend are clean.

Checklist

Checking every item does not guarantee acceptance. Maintainers determine whether
a pull request meets the contribution policy.

• This is a small, concrete change; it is not a feature, refactor, or low-value cleanup.
• I understand that maintainers decide whether the change is obviously correct and trivially verifiable.
• I have read and followed the contribution guidelines.

🤖 Generated with Claude Code

[github-actions]

Thank you for your submission, we really appreciate it. Like many open-source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution. You can sign the CLA by just posting a Pull Request Comment same as the below format.

---

I have read the CLA Document and I hereby sign the CLA

---

_{You can retrigger this bot by commenting recheck in this Pull Request. Posted by the CLA Assistant Lite bot.}

[github-actions]

Thank you for taking the time to contribute. This pull request was automatically closed because:

• The "small, concrete change" confirmation is not checked.
• The "maintainer assessment" confirmation is not checked.
• The "contribution guidelines" confirmation is not checked.
• The patch changes 49 lines; the automatic limit is 30.

Please update the pull request to meet these automatic checks, then reopen it. Passing these checks does not guarantee acceptance; maintainers still determine whether a change is obviously correct and trivially verifiable.

If an exception is appropriate, a maintainer can apply the policy/override label before reopening the pull request. See the contribution guidelines for details.