fix: Handle KeyError in CodecovOption fallback lookups

[sentry]

This PR addresses an AssertionError in CLI validation tests (e.g., validate_empty_upload.py) caused by a KeyError when CodecovOption.get_default attempts to retrieve fallback values from CI adapter mocks.

The root cause was that mock CI adapters, which often use direct dictionary access ([field]), did not always contain all FallbackFieldEnum keys that the CLI command might query. When a missing key was accessed, it raised a KeyError, which was then captured by the CLI runner, leading to a non-zero exit code and failing assertions.

The fix involves wrapping the ci_adapter.get_fallback_value and versioning_system.get_fallback_value calls within CodecovOption.get_default in try/except KeyError blocks. If a KeyError occurs, res is set to None, allowing the fallback mechanism to gracefully continue as if no value was found for that field, rather than crashing.

Fixes CLI-H4

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[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 1726 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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[sentry]

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