fix: Robustify CLI context handling for programmatic invocation

[sentry]

This PR addresses an issue where the CLI could exit with a Click UsageError (SystemExit(2)) when invoked programmatically with an incomplete click.Context.obj object. Specifically, the BrandedOption class would raise a KeyError if ctx.obj was missing the 'branding' key, and commands like upload_coverage and do_upload would raise KeyError if 'versioning_system' or 'codecov_yaml' were missing.

The fix involves:

• Modifying BrandedOption.resolve_envvar_value to safely access ctx.obj and its 'branding' key, providing a default if not present.
• Updating upload_coverage and do_upload commands to use ctx.obj.get() for 'versioning_system' and 'codecov_yaml' to prevent KeyError when these keys might not be set in the context.

Fixes CLI-GK

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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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