fix(chat): handle empty uploaded files case and improve artifact selection logic (#979)

* fix(chat): handle empty uploaded files case and improve artifact selection logic

* Update frontend/src/components/workspace/chats/chat-box.tsx

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* fix: address code review suggestions from PR #979 (#3)

* Initial plan

* fix: address PR #979 review suggestions

- utils.ts: scope (empty) check inside <uploaded_files> tag content
- chat-box.tsx: remove stale `artifacts` from useEffect deps
- context.tsx: wrap select/deselect with useCallback for stable refs
- test: add test_empty_new_files_produces_empty_marker

Co-authored-by: foreleven <4785594+foreleven@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: foreleven <4785594+foreleven@users.noreply.github.com>

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Copilot <198982749+Copilot@users.noreply.github.com>
Co-authored-by: foreleven <4785594+foreleven@users.noreply.github.com>

frontend/src/components/workspace/chats/chat-box.tsx

@@ -34,12 +34,19 @@ const ChatBox: React.FC<{ children: React.ReactNode; threadId: string }> = ({
     setOpen: setArtifactsOpen,
     setArtifacts,
     select: selectArtifact,
+    deselect,
     selectedArtifact,
   } = useArtifacts();
 
   const [autoSelectFirstArtifact, setAutoSelectFirstArtifact] = useState(true);
   useEffect(() => {
     setArtifacts(thread.values.artifacts);
+    if (
+      thread.values.artifacts?.length === 0 ||
+      (selectedArtifact && !thread.values.artifacts?.includes(selectedArtifact))
+    ) {
+      deselect();
+    }
     if (
       env.NEXT_PUBLIC_STATIC_WEBSITE_ONLY === "true" &&
       autoSelectFirstArtifact
@@ -51,7 +58,9 @@ const ChatBox: React.FC<{ children: React.ReactNode; threadId: string }> = ({
     }
   }, [
     autoSelectFirstArtifact,
+    deselect,
     selectArtifact,
+    selectedArtifact,
     setArtifacts,
     thread.values.artifacts,
   ]);