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GH-38477: [Go] Fixing decimal 128 rounding issue (#38478)



### Rationale for this change
Fixing an off-by-one rounding issue with decimal128 by ensuring proper precision handling.

### Are these changes tested?
The test case which reproduced the rounding issue has been added as a unit test.

* Closes: #38477

Authored-by: Matt Topol <zotthewizard@gmail.com>
Signed-off-by: Matt Topol <zotthewizard@gmail.com>