[Variant] Export complete Arrow Fields through the native FFI boundary

[peterxcli]

Problem

The native Arrow C Data exporter constructs FFI_ArrowSchema from ArrayData.data_type(). That preserves the physical datatype but drops the top-level Field name, nullability, and metadata. Logical types represented by Field metadata cannot survive that boundary reliably.

Parent: #5546

Solution

• Pass the corresponding RecordBatch Field to every native array-export call.
• Construct FFI_ArrowSchema from that Field.
• Keep FFI_ArrowArray export unchanged.
• Ensure offset-normalized arrays still use the original output Field.
• Substitute embedded NUL only in the exported C-string name while preserving datatype, nullability, and metadata.

This task must not add Variant scan admission or Variant-specific schema inference.

Tests

• Top-level Field name, nullability, and metadata survive schema export.
• Offset-normalized arrays retain the original Field.
• A top-level name containing NUL does not panic or lose metadata.
• Existing non-Variant array exports remain unchanged.

[peterxcli]

take