[python] Align exact VARIANT integer and decimal types

[XiaoHongbo-Hope]

Follow up on #9147.

• Read VARIANT LONG values through any signed Arrow integer width, matching the integer types accepted by variant_replace.
• Reject negative-scale DECIMAL types before encoding or extraction instead of normalizing them to scale 0.
• Add round-trip and rejection tests.

Validation: 136 passed, 23 subtests passed; flake8 and git diff --check passed.

[JingsongLi]

+1