Deduplicate Concat string and binary builders in datafusion-functions #26304

Is your feature request related to a problem or challenge?

datafusion-functions has two near-identical sets of Concat builders, about 230 lines each:

• binaries.rs#L28-L257
• strings.rs#L67-L311

Both contain an offset-based builder and a view builder. The with_capacity, append_offset and finish bodies and the 7-arm ColumnarValueRef match in write differ only in the target array type, a UTF-8 check on scalars, and .as_bytes() on string values. The view builders' append_offset (make_view, the 12-byte inline threshold and the u32 offset overflow check) is identical in both, so any fix to the view layout logic has to be made twice.

Describe the solution you'd like

• Introduce one byte-oriented builder parameterised over the output array type (a small trait providing the data type, the final array construction, and a scalar-validation hook).
• Add a helper on ColumnarValueRef such as value_bytes(i) -> Option<&[u8]> so the 7-arm match collapses to one call.
• Share the view-append logic between ConcatBinaryViewBuilder and ConcatStringViewBuilder, keeping only the type-specific finish() and its SAFETY comments separate.

Describe alternatives you've considered

Share only the view-builder append_offset, which is the most delicate duplicated piece.

Additional context

Line references are pinned to commit 0d79252.