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GH-39045: [C++][Acero] union node output batches should be unordered (#39046)

### Rationale for this change
Acero's union node produce duplicated batch index if having multiple ordered input nodes, which makes down stream nodes unable to process these batches if ordering is a concern. This PR tries to address this issue.

### What changes are included in this PR?
This PR fixes this issue by setting the index to unsequenced if the order cannot be guaranteed.

### Are these changes tested?
Yes

### Are there any user-facing changes?
No
* Closes: #39045

Authored-by: Yue Ni <niyue.com@gmail.com>
Signed-off-by: Weston Pace <weston.pace@gmail.com>