Fix MySQL expression type inference for numeric division

[rubensantoniorosa2704]

This PR improves MySQL expression type inference for simple numeric expressions, with a focus on division.

In November 2025 I started working on this issue but closed my previous attempt because I felt I was changing too much at once. This is a smaller, more focused version of that work, and it’s my first non-trivial contribution to this project.

What this PR does:

• Fixes the mapping of MySQL division opcodes so that / and DIV are distinguished correctly.
• Introduces a small internal type representation (Kind/Type) for MySQL expression inference.
• Adds inferExprType for MySQL and uses it in output_columns only when inference succeeds, preserving the original fallback behavior otherwise.
• Implements minimal, conservative rules for / so that:
  • FLOAT / is inferred as FLOAT
  • INT / is inferred as DECIMAL
  • nullability is propagated from the operands

The goal is not to provide a complete expression type system, but to address the concrete bug with FLOAT NULL columns and division while leaving clear extension points for future work.

I would really appreciate feedback on:

• Whether the internal Type/Kind abstraction and the scope of this change feel appropriate for the project.
• Any edge cases or additional tests you think should be covered.
• Style/naming/preferences that would make this easier to maintain.

I’m happy to adjust the approach, change the design if you’d prefer a different direction, add more tests, or explain any part of the implementation if something is unclear.

Fixes #4153