[R] array_to_vector() lacks coverage for several core Arrow types

[thisisnic]

Describe the enhancement requested

The R conversion path in r/src/array_to_vector.cpp does not currently handle several built-in Arrow types that exist in the C++ type system.

Core types missing from Converter::Make():

• STRING_VIEW
• BINARY_VIEW
• LIST_VIEW
• LARGE_LIST_VIEW
• RUN_END_ENCODED
• SPARSE_UNION
• DENSE_UNION
• INTERVAL_MONTHS
• INTERVAL_DAY_TIME
• INTERVAL_MONTH_DAY_NANO

Note: extension types like UUID, JSON, BOOL8, OPAQUE, and FIXED_SHAPE_TENSOR are already handled generically via the Type::EXTENSION converter path.

Concrete example:

• Converting a dictionary<values=string_view, indices=uint32> column to R currently errors with:
  cannot handle Array of type <utf8_view>

I am working on STRING_VIEW separately because it shows up in Polars interop, but this issue is to track the broader missing-type coverage.

The likely resolution is not identical for every type:

• Some should convert directly to an R representation (e.g. view types can cast to their non-view equivalents)
• Some may need new R-side representations (e.g. intervals, unions)
• Some may need to error explicitly with an informative message if there is no sensible R mapping

Component(s)

R

[thisisnic]

@jonkeane This came out of a 🤖 session I had recently - does that look right to you?

[jonkeane]

Yeah, one or two of them might actually be covered by a different path, but I know the interval ones weren't bound