fix(workspace): use sanitized filename for non-ASCII Content-Disposition

The RFC5987 encoding path was using the original filename instead of
the sanitized version, bypassing CR/LF/null byte removal.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

autogpt_platform/backend/backend/api/features/workspace/routes.py

@@ -33,7 +33,7 @@ def _sanitize_filename_for_header(filename: str) -> str:
         return f'attachment; filename="{sanitized}"'
     except UnicodeEncodeError:
         # Use RFC5987 encoding for UTF-8 filenames
-        encoded = quote(filename, safe="")
+        encoded = quote(sanitized, safe="")
         return f"attachment; filename*=UTF-8''{encoded}"