[VL] Support ANSI mode decimal Add/Subtract with checked overflow

[n0r0shi]

Summary

• Routes decimal Add and Subtract to Velox's checked_add and checked_subtract when ANSI mode is enabled (nullOnOverflow = false in CheckOverflow), so overflow throws instead of returning null.
• Adds TRY mode support for decimal Add/Subtract using the try(checked_add/checked_subtract) pattern, consistent with how integer TRY arithmetic is already handled.
• Previously, TRY mode for decimal arithmetic relied on a tryCast in CheckOverflowTransformer to detect overflow. This change uses the consistent try(checked_*) pattern instead.

Dependencies

• Depends on feat: Support decimal type for the Spark checked_add and checked_subtract functions facebookincubator/velox#16302 (merged to upstream), which adds checked_add and checked_subtract support for decimal types.
• The commit is on IBM/velox oss-main but not yet on the dft- branch Gluten pins to. The ANSI overflow tests will pass after the next Velox version bump in get-velox.sh.

Test plan

• Normal decimal add/subtract succeeds and matches Spark results
• Overflow on decimal add/subtract throws in ANSI mode
• try_add/try_subtract with decimals returns null on overflow
• Full validation after Velox version bump picks up feat: Support decimal type for the Spark checked_add and checked_subtract functions facebookincubator/velox#16302

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