UV2 Does Not Update When Light Bake Setting Changed

### Tested versions

4.3 Beta 3

### System information

Windows 11, Nvidia RTX4070ti, AMD Ryzen7700x

### Issue description

If a mesh has a custom UV2, switchin from Static to Static Lightmaps (and vice versa) for Light Baking and hitting Reimport does not update the lightmap until the scene is reloaded. The completed bake will act like the setting was never changed.
If the only mesh in the scene has no UV2, switching from Static to Static Lightmaps causes Godot to warn the user that no UV2 is available for lightmapping. The scene must be reloaded to rectify.

### Steps to reproduce

- Open MRP
- Change the Light Bake setting
- Bake the lightmap, no change
- Close and reload the scene
- UV2 will be updated, bake must be redone

### Minimal reproduction project (MRP)

[customlightmapbugv2.zip](https://github.com/user-attachments/files/16208961/customlightmapbugv2.zip)

Start with the attached MRP and reproduce the Light Bake setting change described in the issue, comparing the result before and after reloading the scene. Done means changing between Static and Static Lightmaps updates UV2 and the bake behavior without reloading, including the expected warning when no UV2 is available.

Written by the indexing model from the issue text.