Fix MySQL Mathematical Expression Type Inference

[rubensantoniorosa2704]

This implements type inference for MySQL mathematical expressions. Before this change, queries with arithmetic operations would generate interface{} types, which meant no compile-time type safety. Now the compiler properly infers types based on the operands and operators.

Before:

type ListTestRow struct {
    A1Float interface{} `json:"a1_float"`  // was interface{}
}

After:

type ListTestRow struct {
    A1Float string `json:"a1_float"`  // now string (MySQL DECIMAL)
}

What changed

• Added inferExprType() that recursively analyzes SQL expressions to determine their types
• Handles column references, constants, binary operations, and type casts
• MySQL-specific type rules: division returns decimal, float operations return float, nullability propagates correctly
• Enhanced COALESCE to use type inference instead of always returning interface{}
• Added "div" and "mod" to the mathematical operators list (MySQL uses symbolic names instead of / and %)

Notes

The implementation is MySQL-specific for now. PostgreSQL and SQLite return nil from combineGenericTypes() to maintain current behavior. There's a TODO comment about refactoring IsMathematicalOperator() to be engine-specific.

Closes #4153