Python apps without the template pain points

[davidfowl]

When building a python app from scratch using the python integration there are some rough edges:

When not using uv:

• ❌ No virtual environment is created. The error you get is an obscure DCP error saying that .venv/Scripts/python cannot be found.
  • ❌ There's no instruction about how to get out of this state. We need to do this Add virtual environment validation notification for Python resources #12086 (which is similar to what dev tunnels does). To explain what the user should do.
  • ❌ When you attempt to build the python app in this state, it will generate an empty Dockerfile and fail with an obscure error.

When using uv:

• ❌ If uv is not installed, we need to tell the user to install it (we don't do this yet).
• ✅ Building container images works