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.

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• This is a small, concrete change; it is not a feature, refactor, or low-value cleanup.
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