Schema: require a nonzero coefficient in RationalImpedance polynomial arrays #834

The schema only enforces minItems: 1 on Numerator/Denominator, so validate-config accepts an all-zero polynomial (e.g. "Numerator": [0.0, 0.0]) that the config parser then rejects at solve time. Add to both arrays in $defs/RationalImpedance:

"contains": { "not": { "const": 0 } }

The embedded nlohmann-json-schema-validator supports contains: rejects [0.0, 0.0] and [0, 0], accepts any array with a nonzero entry. Needs a SchemaVer patch bump. Follow-up to #770, punted from #792.