fix: add explicit IsADirectoryError for local directory paths

[pipe1os]

Summary

This PR adds a dedicated IsADirectoryError check in the CLI path resolution to provide a helpful error message when the user attempts to analyze a local directory rather than a specific checkpoint file.

Motivation & Context

Currently, if a user points the CLI to a local directory (e.g., ./my_model/), the tool evaluates os.path.exists as true, but because the directory doesn't have an extension like .safetensors, it falls through to the final else block. The user then receives a confusing error. Adding a dedicated check for directories clarifies the necessary input format, improving DX.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactoring (no functional changes, no api changes)
• Documentation update

Checklist

• My code follows the code style of this project.
• My commit messages follow the Conventional Commits format, are lowercase, imperative, and specific.
• I have updated the documentation accordingly (if applicable).
• I have added tests to cover my changes.
• All new and existing tests passed.