[C++][Feather] Avoid reading outside sliced bitmap buffers in V1 writer #50762

Describe the bug

The Feather V1 writer shifts bitmap bytes when serializing a sliced Boolean
array or validity bitmap. On the final output byte, the shift loop
unconditionally reads the next source byte even when the logical slice ends in
the current byte.

A valid ArrayData backed by an exact-sized Buffer therefore causes a
one-byte heap-buffer-overflow read. The adjacent bit is also folded into the
unused bits of the serialized bitmap byte.

Component

C++ IPC / Feather V1 writer.

Version

Reproduced on current main at
35c5ffd12173284406e4a2c86405415444e596d7.

Minimal reproduction

auto owner = std::make_unique<uint8_t[]>(1);
owner[0] = 0x03;
auto values = std::make_shared<arrow::Buffer>(owner.get(), 1);

auto data = arrow::ArrayData::Make(arrow::boolean(), 1, {nullptr, values},
                                   /*null_count=*/0, /*offset=*/1);
auto array = arrow::MakeArray(data);
auto table = arrow::Table::Make(
    arrow::schema({arrow::field("flag", arrow::boolean())}),
    {std::make_shared<arrow::ChunkedArray>(array)});

ARROW_ASSIGN_OR_RAISE(auto sink, arrow::io::BufferOutputStream::Create());
auto properties = arrow::ipc::feather::WriteProperties::DefaultsV1();
ARROW_RETURN_NOT_OK(
    arrow::ipc::feather::WriteTable(*table, sink.get(), properties));

Under ASan, the write deterministically reports a one-byte
heap-buffer-overflow read in WritePaddedWithOffset in
cpp/src/arrow/ipc/feather.cc.

The same harness with offset = 0 completes without a sanitizer error.

Expected behavior

The writer should read only the bitmap bytes required by the logical slice and
clear unused trailing bits in the serialized bitmap byte.