fix: prevent ValueError when accessing CliRunner.stderr

[sentry]

The click.testing.CliRunner class defaults to mix_stderr=True, which means standard error is merged into standard output. When mix_stderr is True, the stderr_bytes attribute of the Result object remains None.

Attempting to access result.stderr when result.stderr_bytes is None raises a ValueError: stderr not separately captured.

This fix updates all instantiations of CliRunner() in the test suite to CliRunner(mix_stderr=False). This ensures that standard error is always captured separately, preventing the ValueError when result.stderr is accessed.

Fixes CLI-FR