fix: treat a cancelled configcheck as an error, not a passing check

[aa1ex]

Problem

While waiting for a configcheck result, a cancelled context is reported as no error at all. The pipeline reconciler reads that as a passing check and writes a success status, so a config that was never validated is published as validated.

Fix

Return the cancellation as an error instead. A check interrupted by a shutdown now ends with an error in the log rather than a silent pass.

Verification

A unit test cancels the context while the check is waiting and requires an error carrying context.Canceled; it was checked by putting the old return "", nil back and watching the test fail.