chore: import upstream snapshot with attribution

showcase/integrations/langgraph-python/public/demo-files/README.md

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+# Demo Files — Multimodal Demo
+
+This directory bundles sample files referenced by the `/demos/multimodal` page.
+
+Required files (must be committed as binaries; see `.gitattributes` at repo root):
+
+- `sample.png` — a small (< 50 KB) PNG that the "Try with sample image" button
+  injects into the chat. A CopilotKit logo or other recognizable brand mark
+  works well so the vision-capable agent has something to describe.
+- `sample.pdf` — a small (< 50 KB) one-page PDF that the "Try with sample PDF"
+  button injects. The content must mention "CopilotKit" so E2E soft
+  assertions hold (e.g. a one-page export of the CopilotKit quickstart).
+
+The page at `src/app/demos/multimodal/page.tsx` fetches these via the public
+path (`/demo-files/sample.png`, `/demo-files/sample.pdf`), wraps the fetched
+blob in a `File`, and routes it through the same `AttachmentsConfig.onUpload`
+callback the paperclip button uses — so the sample path exercises the exact
+same queueing code as a real user upload.
+
+If these files are missing, the demo page still renders but the sample
+buttons will surface a fetch error. The paperclip / drag-and-drop paths
+continue to work without them.

showcase/integrations/langgraph-python/public/demo-files/sample.pdf

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