fix(frontend): buffer workspace file downloads to prevent truncation (#12349)

## Summary
- Workspace file downloads (images, CSVs, etc.) were silently truncated
(~10 KB lost from the end) when served through the Next.js proxy
- Root cause: `new NextResponse(response.body)` passes a
`ReadableStream` directly, which Next.js / Vercel silently truncates for
larger files
- Fix: fully buffer with `response.arrayBuffer()` before forwarding, and
set `Content-Length` from the actual buffer size
- Keeps the auth proxy intact — no signed URLs (which would be public
and expire, breaking chat history)

## Root cause verification
Confirmed locally on session `080f27f9-0379-4085-a67a-ee34cc40cd62`:
- Backend `write_workspace_file` logs **978,831 bytes** written
- Direct backend download (`curl
localhost:8006/api/workspace/files/.../download`): **978,831 bytes** ✅
- Browser download through Next.js proxy: **truncated** ❌

## Why not signed URLs?
- Signed URLs are effectively public — anyone with the link can download
the file (privacy concern)
- Signed URLs expire, but chat history persists — reopening a
conversation later would show broken downloads
- Buffering is fine: workspace files are capped at 100 MB, Vercel
function memory is 1 GB

## Related
- Discord thread: `#Truncated File Bug` channel
- Related PR #12319 (signed URL approach) — this fix is simpler and
preserves auth

## Test plan
- [ ] Download a workspace file (CSV, PNG, any type) through the copilot
UI
- [ ] Verify downloaded file size matches the original
- [ ] Verify PNGs open correctly and CSVs have all rows

cc @Swiftyos @uberdot @AdarshRawat1

autogpt_platform/frontend/src/app/api/proxy/[...path]/route.ts

@@ -57,27 +57,25 @@ async function handleWorkspaceDownload(
     );
   }
 
-  // Get the content type from the backend response
+  // Fully buffer the response before forwarding.  Passing response.body as a
+  // ReadableStream causes silent truncation in Next.js / Vercel — the last
+  // ~10 KB of larger files are dropped, corrupting PNGs and truncating CSVs.
+  const buffer = await response.arrayBuffer();
+
   const contentType =
     response.headers.get("Content-Type") || "application/octet-stream";
   const contentDisposition = response.headers.get("Content-Disposition");
 
-  // Stream the response body
   const responseHeaders: Record<string, string> = {
     "Content-Type": contentType,
+    "Content-Length": String(buffer.byteLength),
   };
 
   if (contentDisposition) {
     responseHeaders["Content-Disposition"] = contentDisposition;
   }
 
-  const contentLength = response.headers.get("Content-Length");
-  if (contentLength) {
-    responseHeaders["Content-Length"] = contentLength;
-  }
-
-  // Stream the response body directly instead of buffering in memory
-  return new NextResponse(response.body, {
+  return new NextResponse(buffer, {
     status: 200,
     headers: responseHeaders,
   });