CLI validate accepts node source paths outside the source directory #659

Hey,

I came across an inconsistency between the validate and build commands while testing source file paths.
I created a workflow that references ../outside.py from a node. The file exists, but it's located outside the source directory.

When I ran concore validate , it reported that the workflow was valid and ready to run. But when I tried to build the same workflow, the build failed with:

ValueError: Unsafe Node source file ../outside.py: invalid path segment.

It looks like validation only checks whether the file exists, while the build command also checks whether the path is safe.

Expected [behaviour]
concore validate should catch this and report the invalid source path before the user attempts to build the workflow.

Steps to reproduce

1. Create a source directory
2. Place outside.py in its parent directory, outside the source directory
3. Reference it as ../outside.py in a workflow node
4. Run concore validate followed by concore build

I tested this on Windows using the current dev branch and was able to reproduce it consistently.