Shredded variant readers throw InvalidCastException for large_binary and binary_view storage #448

Describe the bug, including details regarding any error messages, version, and platform.

VariantArray accepts binary, large_binary or binary_view for its metadata and value fields, at the top level and inside shredded element groups (VariantExtensionDefinition.FindBinaryFieldIndex, ShredSchema.IsElementGroupStruct). ShredSchema.FromArrowType also maps large_utf8 and large_binary typed_value columns.

The shredding-aware readers in Apache.Arrow.Operations.Shredding (ShreddedVariant, ShreddedObject, ShreddedArray) cast those columns to BinaryArray / StringArray, so any other representation throws InvalidCastException. This affects:

• GetLogicalVariantValue on any such column, shredded or not. For an unshredded large_binary column the core VariantArray.GetVariantValue works, but GetLogicalVariantValue throws.
• ShreddedVariant.GetString() / GetBinaryBytes() on large_utf8 / large_binary typed columns.
• Residual reads in ShreddedObject / ShreddedArray, TryGetResidualReader, etc.

Expected: the readers accept every storage type that VariantArray and ShredSchema accept.

Found while reviewing #447, whose new array-level entry points go through these readers.

Component(s)

C#