fix: handle invalid slug gracefully to prevent CLI crash

[sentry]

This PR addresses issue CLI-2N, where the Codecov CLI would crash with an unhandled ValueError: The provided slug is invalid when the slug argument was either not provided by the user or could not be auto-detected from the environment.

Root Cause:
The encode_slug function in codecov_cli/helpers/encoder.py raises a ValueError for invalid or None slugs. Previously, this ValueError was not caught by the calling service functions (e.g., create_commit_logic), leading to an unhandled exception and a CLI crash.

Solution:

1. Introduced a new helper function safe_encode_slug in codecov_cli/helpers/encoder.py. This function wraps encode_slug in a try-except block, logs a user-friendly error message, and returns None if the slug is invalid.
2. Modified all call sites of encode_slug across various service modules (e.g., commit, report, empty_upload, upload_completion, upload_sender, get_report_results) to use safe_encode_slug.
3. Added a check after calling safe_encode_slug: if it returns None, a click.UsageError is raised. This ensures the CLI exits cleanly with a clear error message to the user, rather than crashing.

Fixes CLI-2N

[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
723	1	722	1

View the full list of 1 ❄️ flaky test(s)

api.temp.calculator.test_calculator::test_divide

Flake rate in main: 100.00% (Passed 0 times, Failed 1625 times)

Stack Traces | 0.001s run time

def
                test_divide():
                > assert Calculator.divide(1, 2) == 0.5
                E assert 1.0 == 0.5
                E + where 1.0 = <function Calculator.divide at 0x104c9eb90>(1, 2)
                E + where <function Calculator.divide at 0x104c9eb90> = Calculator.divide
                .../temp/calculator/test_calculator.py:30: AssertionError

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