fix: handle missing/invalid slug with user-friendly error

[sentry]

This PR addresses CLI-33, where users encountered an unhandled ValueError when the --slug parameter was missing or invalid, particularly in CI environments where auto-detection might fail (e.g., Bitbucket).

Problem:

• The encode_slug function in codecov_cli/helpers/encoder.py would raise a ValueError if the provided slug was None or malformed.
• This ValueError was not caught by upstream callers like create_commit_logic, leading to an unhandled exception and a crash of the CLI process.
• Users received a generic Python traceback instead of actionable feedback.

Solution:

• Replaced the ValueError in encode_slug with click.UsageError. This ensures that invalid slug formats result in a user-friendly error message and a clean exit from the CLI.
• Added an explicit check for a None or empty slug in create_commit_logic within codecov_cli/services/commit/__init__.py. If the slug is missing, a click.UsageError is raised, guiding the user to provide the --slug owner/repo option when auto-detection is not possible.

Impact:

• Prevents CLI crashes due to missing or invalid slugs.
• Provides clear, actionable error messages to users, improving the overall user experience and reducing support inquiries.

Fixes CLI-33

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