MAINT add tests for build scripts

[romanlutz]

It would be useful to have tests for the build scripts on all platforms.

Originally posted by @romanlutz in #1569 (comment)

[tejas0077]

Hi @romanlutz I'd like to take this on. Looking at #1569, I can see the context is testing prepare_package.py, particularly the npm lookup logic added for Windows compatibility. I'll write cross-platform tests covering the npm path resolution, subprocess handling, and failure scenarios. Will open a draft PR shortly happy to get your sign-off on scope before going too deep.

[romanlutz]

We can expand to other build scripts as it makes sense, too.

[tejas0077]

Hi @romanlutz — PR is up at #1642. 46 unit tests across 4 build scripts, all passing locally.