fix: list_kernel_files returns 0 files (issue #51)

[1chooo]

Summary

• Root cause 1 — wrong API: list_kernel_files returns kernel source files, not run output artifacts. Users expecting output files (CSVs, model weights, etc.) should use list_kernel_session_output instead. Added clear docstrings to both methods and both request types explaining the distinction.
• Root cause 2 — silent empty response: user_name and kernel_slug default to "" and are silently omitted from the request body, causing the server to return "files": [] with no error. Both methods now raise ValueError with a descriptive message (including a pointer to the correct API) when either field is missing.
• Root cause 3 — latent crash on null response: If the server ever returns "files": null, set_from_dict in kaggle_object.py would crash with TypeError inside ListSerializer. None is now treated the same as the default empty list.

Test plan

• Call list_kernel_files without setting user_name/kernel_slug — expect ValueError with a message mentioning list_kernel_session_output.
• Call list_kernel_session_output without setting user_name/kernel_slug — expect ValueError with a message mentioning list_kernel_files.
• Call list_kernel_files with valid user_name and kernel_slug — expect normal file list response.
• Simulate a server response with "files": null — expect empty list, no crash.

Closes #51

Made with Cursor