fix: prevent unhandled ValueError for invalid slug - #781

This PR addresses issue CLI-1D, where the codecov create-commit command would crash with an unhandled ValueError: The provided slug is invalid if no slug was provided via --slug and no CI environment variables could supply one.

Root Cause:
The encode_slug function in codecov_cli/helpers/encoder.py was raising a bare ValueError when the input slug was None or did not conform to the expected format. As ValueError is not a click.ClickException, the Click framework could not catch and handle it gracefully, leading to an unhandled exception and CLI crash.

Solution:

• Modified codecov_cli/helpers/encoder.py to import click.
• Updated encode_slug to raise click.ClickException instead of ValueError when the slug is None or invalid. An explicit check for None provides a more specific error message.
• Updated the corresponding test test_encode_invalid_slug in tests/helpers/test_encoder.py to expect click.ClickException.

Impact:
The CLI will now display a user-friendly error message and exit gracefully when a slug is missing or invalid, preventing unhandled exceptions and improving user experience.

Fixes CLI-1D

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