fix: preserve high-precision DECIMAL parameters

[skhe]

Changes

• Parse DECIMAL parameters as exact base-10 digits instead of a default-precision binary float.
• Reject values that exceed the decimal wire format rather than silently truncating.
• Add real-DM regression cases for Decimal, text, negative, and exponent forms of DECIMAL(30,8).

Verification

• Local official DM8 ARM64: 108 real-database tests passed on Python 3.10.
• Focused DECIMAL cases: 4 passed on each of Python 3.9, 3.11, 3.12, and 3.13.
• Go bridge build, go test, and git diff --check passed.