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GH-40693: [Go] Fix Decimal type precision loss on GetOneForMarshal (#40694)

### Rationale for this change

Loss of precision when using `GetOneForMarshal` on `Decimal128` and `Decimal256`

### What changes are included in this PR?

Fixes for precision loss with `DecimalType.GetOneForMarshal`

* GitHub Issue: #40693

Lead-authored-by: Herman Schaaf <hermanschaaf@gmail.com>
Co-authored-by: Kemal Hadimli <disq@users.noreply.github.com>
Signed-off-by: Matt Topol <zotthewizard@gmail.com>