build: perform pdm sync before running commands

[Connor1996]

I encountered a build error when running pdm run build-ext, then I finally found that it's due to old mlx version in venv which is not consistent with the version in pdm.lock.

Should perform pdm sync after updating pdm.lock, so adding a pre_run hook to run it automatically before running any command.

[skyzh]

I wonder if we should ask students to run that before each test in the writeup. It feels like silently sync will make things confusingly slow..?

[Connor1996]

pdm sync is quite fast when there is nothing to update