fix: prevent ValueError when slug is missing in upload

[sentry]

This PR addresses an issue where the upload command would raise a ValueError: The provided slug is invalid if no --slug argument was provided and the CI adapter could not auto-detect a slug. The None value for slug was being passed to encode_slug(), which then failed validation.

The fix introduces an early validation check within the do_upload_logic function. If the slug parameter is None or otherwise invalid, a click.ClickException is now raised with a clear, user-friendly message. This guides the user to provide the --slug option in the correct format, preventing the unhelpful ValueError and improving the overall user experience.

Fixes CLI-GH

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