fix: modify streaming fastapi to support vercel/ai

templates/simple/fastapi/README-template.md

templates/simple/fastapi/app/api/routers/chat.py

templates/simple/fastapi/app/utils/__init__.py

templates/simple/fastapi/app/utils/index.py

templates/simple/fastapi/main.py

templates/streaming/fastapi/README-template.md

templates/streaming/fastapi/app/api/routers/chat.py

templates/streaming/fastapi/app/utils/__init__.py

templates/streaming/fastapi/app/utils/index.py

templates/streaming/fastapi/app/utils/json.py

templates/streaming/fastapi/main.py

templates/streaming/fastapi/pyproject.toml