Fix serializer typing

[wk9874]

Fix GridMetrics serializer typing

Python Version(s) Tested: 3.10

Operating System(s): Ubuntu 22.04

📝 Summary

serialize_array in GridMetricSet has the wrong typing - doesnt include list[list[float]] as an option in the inputs and return type

🔍 Diagnosis

Lots of these warnings printed:

  PydanticSerializationUnexpectedValue(Expected `float` - serialized value may not be as expected [input_value=[20.0, 20.0, 20.0, 20.0, ... 20.0, 20.0, 20.0, 20.0], input_type=list])

🔄 Changes

Add correct typing

✔️ Checklist

• Unit and integration tests passing.
• Pre-commit hooks passing.
• Quality checks passing.