[python][cpp] [bindings] Nested ROW field descriptions are dropped by bindings

[fresh-borzoni]

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Description

Core round-trips per-field ROW descriptions (DataField.description, via json_serde), but the python/cpp bindings don't surface them

Willingness to contribute

• I'm willing to submit a PR!

[iamrajatrana]

Hi, I’d like to work on this issue.

I traced it back to #3603. My understanding is that nested DataField.description is preserved by the Rust core but omitted by the Python and C++ binding representations. In C++, the FFI type node carries field_name but no description, and conversion constructs the field with DataField::new(field_name, field_type, None).

I propose adding optional field descriptions to the binding/FFI representations, preserving them in both conversion directions, and adding nested ROW round-trip tests. I’ll aim to keep the existing description-less construction APIs backward compatible.

@fresh-borzoni @charlesdong1991, does this approach and scope sound right? Is there a preferred public representation for description-bearing nested fields in the Python and C++ APIs?