Validate porter file/dir conflicts

[kichristensen]

What does this change

Adds validation during porter build to detect if a file or directory named porter exists in the bundle directory. Returns a clear error message before the confusing runtime failure occurs.

Before: Bundle build succeeds but installation fails with:

could not create outputs directory /cnab/app/porter/outputs: mkdir /cnab/app/porter: not a directory

After: Build fails early with clear error:

a file or directory named "porter" exists in /path/to/bundle, which will conflict with Porter's internal directory structure. Please rename or remove it, or add it to .dockerignore

What issue does it fix

Closes #3536

Notes for the reviewer

The fix adds validation in BuildOptions.Validate() to check for the conflicting porter file/directory before the build process copies files into the bundle image. Two test cases cover both file and directory scenarios.

Checklist

• Did you write tests?
• Did you write documentation?
• Did you change porter.yaml or a storage document record? Update the corresponding schema file.
• If this is your first pull request, please add your name to the bottom of our Contributors list. Thank you for making Porter better! 🙇‍♀️