fix(mysql): decode DECIMAL columns into integer types correctly

[Map130]

Problem

Decoding a MySQL DECIMAL/NEWDECIMAL column into an integer type silently
corrupted the value. MySQL sends DECIMAL as ASCII text even in the binary
protocol, but the integer decoders read those bytes as a little-endian
integer, so decoding e.g. SUM(int_col) into u64 via an explicit type
override or try_get_unchecked produced garbage: the bytes b"13"
(0x31 0x33) decoded to 0x3331 = 13105 instead of 13.

Fix

int_decode/uint_decode now detect DECIMAL/NEWDECIMAL columns and parse
the textual value, requiring it to be integral:

• integral DECIMAL → correct integer value;
• fractional DECIMAL → a clean has a fractional part error instead of a
  silently corrupted result;
• negative DECIMAL → decodes into signed integers, errors for unsigned.

Type::compatible is deliberately left unchanged, so macro inference still
maps DECIMAL columns to Decimal/BigDecimal and does not lossily infer
u64 for SUM().

Tests

Added decode_integer_from_decimal in tests/mysql/types.rs covering the
correct-value, fractional-error, and negative cases. Verified against a
live MySQL 8.0 with and without the rust_decimal/bigdecimal features;
existing mysql-types and mysql-macros suites pass.