fix(mysql): support DECIMAL/NEWDECIMAL decoding in tauri-plugin-sql

[AbdenourTadjer33]

Summary

This PR fixes MySQL decimal column decoding in tauri-plugin-sql.

Before this change, selecting rows containing DECIMAL columns could fail with:

• unsupported datatype: DECIMAL (when type was unhandled), or
• null decoded values (when trying incompatible string decoding).

Root cause

src/decode/mysql.rs did not properly decode MySQL decimal types through a compatible SQLx type.

DECIMAL / NEWDECIMAL require decimal-aware decoding in SQLx, not plain String decode compatibility.

Changes

1) MySQL decode support

• Updated src/decode/mysql.rs
• Added handling for:
  • DECIMAL
  • NEWDECIMAL
• Decode path:
  • try_decode::<BigDecimal>()
  • serialize as JSON string via to_string()

This preserves precision and avoids float conversion loss.

2) Dependency feature

• Updated plugin Cargo.toml
• Enabled SQLx feature:
  • bigdecimal

Why JSON string output?

Decimal values are precision-sensitive (money/tax/rates).
Returning as string avoids precision loss and matches common JS/TS handling for fixed-point values.

Compatibility

• No breaking API changes.
• Existing consumers receive decimal values as JSON strings instead of decode errors/nulls.
• Behavior for non-decimal MySQL types is unchanged.

Validation

• Build/check passes with patched plugin.
• Queries against decimal columns now return non-null values.
• No regressions observed in non-decimal query paths.