After the last update, the Undo button is not appearing in the chat screen.

I enter the following example code into the chat screen, the code is applied, but the Undo button does not appear. The Undo button is also grayed out in the Edit menu. I don't know if this is due to the update or a settings issue, as this is my first time using Xcode. Before the update, there were no problems; the Undo button would appear right next to the chat screen after the code was applied, but now it's gone. Can you help me with this?

Edit only:
MetalPortfolio/MetalPortfolio/Models/Transaction.swift

Goal:
Make Transaction decoding backward-compatible so older saved transactions do not fail to load when newer fields are missing.

Critical rules:
- Do not edit any other file.
- Do not change Transaction stored property names.
- Do not change encoding format.
- Do not change UI.
- Do not change storage/export/import logic yet.
- Do not change calculations.
- Keep existing behavior for valid current JSON.

Implementation:
In Transaction.init(from:), change only optional/backward-compatible decoding where safe.

Required:
- fee should decode with default 0 if missing:
  fee = try container.decodeIfPresent(Double.self, forKey: .fee) ?? 0

Optional if consistent with existing defaults:
- unit can keep decodeIfPresent fallback.
- currency can keep decodeIfPresent fallback.

Do not make core required fields optional:
- id
- type
- asset
- quantity
- price
- date

Return only:
- which file changed
- exact decoding behavior changed