fix: replace local vendor folders with published stable packages

- Remove local ag_ui_langgraph/ and copilotkit/ vendor directories
- Use ag-ui-langgraph==0.0.28 and copilotkit==0.1.82 from PyPI
- Clean up Dockerfile (no longer needs TestPyPI workarounds)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: tylerslaton