feat: improve MySQL mathematical expression type inference

[rubensantoniorosa2704]

Problem

MySQL mathematical expressions like (a1 / 1024) were being generated as interface{} instead of proper types like sql.NullFloat64.

Root Cause

• The IsMathematicalOperator function only supported PostgreSQL operators
• MySQL's TiDB parser converts / to "div" operator, which wasn't recognized
• Expression type inference was hardcoded to "int" for all mathematical operations

Solution

1. Added MySQL "div" operator support to IsMathematicalOperator
2. Implemented intelligent type inference that:
   • Analyzes operand types from table schemas
   • Considers operation type (division → float)
   • Combines types intelligently
   • Supports recursive expressions

Before/After

// Before: type Row struct { A1Float interface{} // ❌ A2Float interface{} // ❌ A3 sql.NullFloat64 // ✅ } // After:  type Row struct { A1Float sql.NullFloat64 // ✅ A2Float sql.NullFloat64 // ✅ A3 sql.NullFloat64 // ✅ }