[C++][Acero] union node output batches should be unordered

[niyue]

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

Acero's union node may have multiple input nodes that have ordered output, so the union node's input batches may contain batch index from previous nodes. However, the union node output doesn't guarantee any order, so it should clear the batch index if it has multiple input nodes, so that downstream node won't be confused by its input when ordering is a concern.

According to the doc for batch index property:

arrow/cpp/src/arrow/compute/exec.h

Lines 223 to 225 in 47dadb0

	/// This index must be strictly monotonic starting at 0 without gaps or
	/// it can be set to kUnsequencedIndex if there is no meaningful order
	int64_t index = kUnsequencedIndex;

The downstream is expected to receive only strictly monotonic starting at 0 without gaps, but for a union node with multiple ordered input nodes, it will produce duplicated batch indexes, which is not expected.

Component(s)

C++