GH-50658: [C++][Dev] Add RunEndEncoded support to gdb_arrow.py

[fenfeng9]

Rationale for this change

The Arrow GDB pretty-printers do not support run-end encoded data types.

What changes are included in this PR?

Add GDB pretty-printer support and tests for run-end encoded types, scalars, and arrays.

Are these changes tested?

Yes.

Are there any user-facing changes?

No.

• GitHub Issue: [C++][Dev] Implement RunEndEncoded support in gdb_arrow.py #50658

[fenfeng9]

Here are some examples from a local GDB session.

RunEndEncodedType

RunEndEncodedType run_end_encoded_type(int32(), utf8());
auto heap_run_end_encoded_type =
    run_end_encoded(int32(), utf8());

(gdb) p run_end_encoded_type
$1 = arrow::run_end_encoded(arrow::int32(), arrow::utf8())

(gdb) p *heap_run_end_encoded_type
$2 = (std::__shared_ptr_access<arrow::DataType, (__gnu_cxx::_Lock_policy)2, false, false>::element_type &) @0x555555f52f10:
     arrow::run_end_encoded(arrow::int32(), arrow::utf8())

RunEndEncodedScalar

auto run_end_encoded_scalar_type =
    run_end_encoded(int32(), utf8());

RunEndEncodedScalar run_end_encoded_scalar{
    MakeScalar("foo"), run_end_encoded_scalar_type};

RunEndEncodedScalar run_end_encoded_scalar_null{
    run_end_encoded_scalar_type};

(gdb) p run_end_encoded_scalar
$3 = arrow::RunEndEncodedScalar of value
     arrow::StringScalar of size 3, value "foo"

(gdb) p run_end_encoded_scalar_null
$4 = arrow::RunEndEncodedScalar of type
     arrow::run_end_encoded(arrow::int32(), arrow::utf8()),
     null value

RunEndEncodedArray

// Encodes ["foo", "foo", null, null, null].
auto run_end_encoded_run_ends =
    SliceArrayFromJSON(int32(), "[2, 5]");
auto run_end_encoded_values =
    SliceArrayFromJSON(utf8(), R"(["foo", null])");

auto heap_run_end_encoded_array =
    *RunEndEncodedArray::Make(
        5, run_end_encoded_run_ends,
        run_end_encoded_values);

auto heap_run_end_encoded_array_sliced =
    heap_run_end_encoded_array->Slice(1, 3);

(gdb) p *heap_run_end_encoded_array
$5 = (std::__shared_ptr_access<arrow::Array, (__gnu_cxx::_Lock_policy)2, false, false>::element_type &) @0x555555fab440:
     arrow::RunEndEncodedArray of type
     arrow::run_end_encoded(arrow::int32(), arrow::utf8()),
     length 5, offset 0, null count 0

(gdb) p *heap_run_end_encoded_array_sliced
$6 = (std::__shared_ptr_access<arrow::Array, (__gnu_cxx::_Lock_policy)2, false, false>::element_type &) @0x555555f7dd60:
     arrow::RunEndEncodedArray of type
     arrow::run_end_encoded(arrow::int32(), arrow::utf8()),
     length 3, offset 1, null count 0

[fenfeng9]

The null count 0 output is expected here and follows the existing ArrayData behavior. It counts only nulls in the top-level array. Run-end encoded arrays store nulls in the values child array, so this example decodes to three nulls even though the printer shows 0.