fix: handle missing 'module'/'class' keys in custom plugin config

[sentry]

This PR addresses a KeyError: 'module' that occurred when a custom plugin was defined in codecov.yml without specifying the module or class keys. Previously, _load_plugin_from_yaml would directly subscript plugin_dict['module'] and plugin_dict['class'], leading to an unhandled KeyError if these keys were absent.

The fix adds an early validation check within _load_plugin_from_yaml. It now verifies the presence of both module and class keys. If either is missing, it logs an informative error message using click.secho and gracefully returns a NoopPlugin(), preventing the CLI from crashing. This approach is consistent with the existing error handling patterns within the function for other plugin loading issues.

Fixes CLI-BX

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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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