Scenes with 3D MultiMeshes print errors on load

### Tested versions

4.3

### System information

Windows 11

### Issue description

https://github.com/godotengine/godot/blob/ee363af0ed65f6ca9a906f6065d0bd9c19dcf9e4/scene/resources/multimesh.cpp#L309 prints an error if the `instance_count` is set before `set_transform_format`. This will be true for any non empty 3D `MultiMesh` stored in a scene file.

### Steps to reproduce

Add a 3D `MultiMesh` with some instances to a scene.
Save scene.
Reopen scene.
` Condition "instance_count > 0" is true.`

### Minimal reproduction project (MRP)

-

Start at scene/resources/multimesh.cpp:309 and reproduce the warning by adding a 3D MultiMesh with instances, saving the scene, and reopening it. Trace the load order for instance_count and set_transform_format, then verify that reopening the saved scene no longer prints the condition error.

Written by the indexing model from the issue text.