Add error handling for session bootstrap failures: Implement a client-safe error response payload in session_bootstrap.py and update Streamlit routes to use this new error handling.

Refactor session bootstrap handling and Streamlit app startup: Update start.sh to launch the app using uvicorn, enhance session bootstrap logic in session_bootstrap.py with retry mechanisms for transient errors, and modify streamlit_app.py to conditionally use a new route for session initialization. Add tests for retry behavior and error handling in session bootstrap payload generation.