Unhelpful error message when ArrayMesh has a shadow_mesh with a different number of surfaces

### Tested versions

Initially found in 4.2.2 (Steam buildid 14084462).
Reproducible in 4.0-stable (after removing type hints on loops).
Reproducible in 4.1-stable (after removing type hints on loops).
Reproducible in 4.3-beta3.

### System information

Godot v4.2.2.stable - Debian GNU/Linux trixie/sid trixie - Wayland - Vulkan (Forward+) - integrated Intel(R) Graphics (RPL-P) () - 13th Gen Intel(R) Core(TM) i7-1360P (16 Threads)

### Issue description

The repro project spams the following errors in the output window:
```
ERROR: Index p_surface_index = 1 is out of bounds (mesh->surface_count = 1).
at: mesh_get_surface (./servers/rendering/renderer_rd/storage_rd/mesh_storage.h:382)
ERROR: Index p_surface_index = 2 is out of bounds (mesh->surface_count = 1).
at: mesh_get_surface (./servers/rendering/renderer_rd/storage_rd/mesh_storage.h:382)
ERROR: Index p_surface_index = 1 is out of bounds (mesh->surface_count = 1).
at: mesh_get_surface (./servers/rendering/renderer_rd/storage_rd/mesh_storage.h:382)
ERROR: Index p_surface_index = 2 is out of bounds (mesh->surface_count = 1).
at: mesh_get_surface (./servers/rendering/renderer_rd/storage_rd/mesh_storage.h:382)
```

There does not appear to be anything wrong visually, but obviously there is something wrong if it's spamming error messages.

### Steps to reproduce

1. Open project
2. Observe output window

If the code in `res://code/terrain/terrain_mesh.gd` is removed, the error still occurs, but only once.

### Minimal reproduction project (MRP)

[voxel.zip](https://github.com/user-attachments/files/16365065/voxel.zip)

Reproduce the output spam with the linked voxel project, then inspect mesh_get_surface in servers/rendering/renderer_rd/storage_rd/mesh_storage.h and the shadow_mesh surface handling. Trace why surface indices exceed the shadow mesh surface count, and make the resulting diagnostic identify the mismatched surface configuration; verify the repro no longer emits unhelpful repeated errors.

Written by the indexing model from the issue text.