Generate fallback Dockerfile for Python apps without UV support

[davidfowl]

Currently, the dotnet/aspire tooling only generates a Dockerfile for Python apps when UV is being used. There is no automatic Dockerfile generation as a fallback when UV is not present.

This issue tracks adding support for generating a default Dockerfile for all Python apps when UV is not being used and no Dockerfile exists. The generated Dockerfile should:

• Detect if a requirements.txt is present and run pip install for dependencies if so.
• Use the same runtime image as the UV workflow for consistency.
• Apply this fallback logic for all Python apps, not just certain scenarios.

Reference: The current implementation at src/Aspire.Hosting.Python/PythonAppResourceBuilderExtensions.cs only generates a Dockerfile when UV is used. This enhancement will ensure Python apps are supported even without UV and that dependencies are installed correctly via pip when requirements.txt is present.

[davidfowl]

For 13, we will provide a good error message.