DYN-8088: DSCore.List.Flatten does not work in PythonNet3

[ivaylo-matov]

This PR addressed DYN-8088.

Fix ListEncoderDecoder so Python iterables decode into real CLR lists:

• build List manually for generic requests, ensuring element types (int, doubles, strings, nested lists) become actual .NET values.
• add conversion helpers for Python int/floats/tuples and guard against returning PyObject wrappers.

Restores legacy behavior so DSCore.List.Flatten under PythonNet3 as they did in CPython.

There will be separate PR to DSDynamo with a test.

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