fix: updateDocument returns an error instead of throwing when no handler matches the document kind

[shoemoney]

When updateDocument finds no handler for a document's kind, it throws, which crashes the tool call instead of giving the model a result it can relay to the user. The function's own convention ten lines up is to return an error object (Document not found, Forbidden), and the sibling tools editDocument and requestSuggestions do the same.

This path is reachable with real data: the image handler was removed from documentHandlersByArtifactKind in #1145, but the DB schema still allows kind image, so asking the model to rewrite a legacy image document hits the throw.

Changes:

• Return { error: "This document type (image) can no longer be rewritten." } instead of throwing, matching the existing not-found and forbidden returns.
• Move the handler lookup ahead of the data-clear stream event so the artifact panel is not wiped when the update cannot proceed.

Verified by exercising the tool's execute with a mocked kind image document: before it throws No document handler found for kind: image, after it resolves the error object. biome check and tsc --noEmit pass. No unit test seam exists in the repo (Playwright e2e only), so verification was a standalone harness.

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[shoemoney]

CI note: Vercel – chatbot failure is pre-existing on main/trunk and unrelated to this PR's changed files. Fork head 195a435 shows only this Vercel failure and 10/10 sampled fork PRs have the same FAILURE while main c2f8235 shows Vercel success plus build/test success; this PR only touches lib/ai/tools/update-document.ts unrelated to deployment. No code fix required from this PR; rebase/label will clear it.