Closes#3192
Root cause
----------
The artifact preview header is driven by a Radix Select whose <SelectValue>
renders the label of the <SelectItem> matching the current value. The option
list was built solely from `artifacts` in ArtifactsContext, which is only
synced from `thread.values.artifacts` (chat-box.tsx). Artifacts surfaced via
the message-layer `present_files` tool call are never written back to
`thread.values.artifacts`, so when such a file is selected its filepath has
no matching <SelectItem>. Radix then renders an empty trigger and the header
filename appears blank, even though the preview body loads correctly.
Fix
---
Compute `artifactOptions` as a defensive union: if the currently selected
filepath is missing from `artifacts`, prepend it so a matching <SelectItem>
always exists. This keeps the header label in sync with the active file
without changing context semantics or coupling the UI to a future
auto-discovery mechanism (see existing TODO in chat-box.tsx).
Tests
-----
Add an e2e case that mocks a thread whose only artifact is delivered via
`present_files` (thread.values.artifacts = []) and asserts both the header
title and preview content render. All 21 e2e tests pass.
