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GH-45254: [C++][Acero] Fix the row offset truncation in row table merge (#45255)



### Rationale for this change

See #45254

### What changes are included in this PR?

First modify the test case to expose the suspecting bug.

Then the fix in source.

### Are these changes tested?

By existing tests.

### Are there any user-facing changes?

None.

* GitHub Issue: #45254

Authored-by: Rossi Sun <zanmato1984@gmail.com>
Signed-off-by: Rossi Sun <zanmato1984@gmail.com>