fix: handle missing slug in send-notifications command

[sentry]

This PR addresses an issue where the send-notifications command would raise a ValueError: The provided slug is invalid if the --slug option was not explicitly provided and the slug could not be auto-detected from CI environment variables.

Previously, a None or empty slug value was passed directly to encode_slug(), which would then raise a generic ValueError.

The fix introduces a check for a missing slug within the upload_completion_logic function (which send-notifications calls). If the slug is missing:

• A clear, actionable error message is logged, guiding the user to either provide --slug owner/repo or ensure their CI environment variables are correctly set for auto-detection.
• If fail_on_error is True, a ValueError with the same descriptive message is raised, causing a clean failure.
• If fail_on_error is False, the error is logged, and the function returns None gracefully, preventing an unhandled exception.

Fixes CLI-FY

[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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