fix: testing that user doesn't pass folder to --cp

[quintesse]

Fixes #2045

Summary by Sourcery

Disallow users from passing directories to the --cp option by validating classpath entries and throwing an error when a directory is detected, and ensure this validation runs across all CLI commands.

Bug Fixes:

• Reject directories passed to --cp and report an illegal argument error

Enhancements:

• Introduce a shared validate() method for DependencyInfoMixin and invoke it in all relevant commands

Tests:

• Add a test to verify that passing a folder to --cp raises an IllegalArgumentException