instance_geometry_set_lightmap does nothing

### Tested versions

Reproducible: v4.5.1.stable.official [f62fdbde1]

### System information

Godot v4.5.1.stable - Windows 10 (build 19045) - Multi-window, 1 monitor - Vulkan (Forward+) - dedicated NVIDIA GeForce RTX 3050 Ti Laptop GPU (NVIDIA; 32.0.15.8183) - AMD Ryzen 5 5600H with Radeon Graphics (12 threads) - 15.34 GiB memory

### Issue description

Setting a lightmap with `RenderingServer.instance_geometry_set_lightmap` does not do anything — mesh instances are rendered without any lightmaps.

Baked scene:

Main scene with instances of the baked scene:

### Steps to reproduce

Copy info from `LightmapGI` node into a separate lightmap (using `lightmap_create`).

### Minimal reproduction project (MRP)

Run main scene and observe blank meshes
[lightmap_gi_for_instances_2026-02-10_18-13-43.zip](https://github.com/user-attachments/files/25214051/lightmap_gi_for_instances_2026-02-10_18-13-43.zip)