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GH-35310: [Go] Incorrect value decimal128 from string (#35311)



### Rationale for this change
Fix edge case of rounding when parsing a decimal128 value from string with a high precision.

### What changes are included in this PR?
Adding a simple 0.5 rounding based on the signbit before we truncate the value to an integer.

### Are these changes tested?
Yes, an edge case test case was added.

### Are there any user-facing changes?
Some values that were previously incorrect will now be correct.

* Closes: #35310

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