Runtime NavigationMesh baking on thread broken due to new thread guards

[smix8]

Godot version

da21cb7

System information

Windows 10

Issue description

The new thread guards added with the recent SceneTree thread groups from pr #75901 break the runtime baking of navigation meshes on a thread in debug builds.

The navigation mesh baking process first parses the child nodes recursive to gather source geometry for the baking. This process is now entirely blocked by the thread guards in debug builds. The guards make the SceneTree nodes inaccessible from threads. The navigation mesh baking process now suddenly has zero data to work with.

Workaround is to use NavigationRegion3D.bake_navigation_mesh(false) or use the NavigationMeshGenerator.bake() directly with the NavigationMesh or to do as suggested by the error msg and call deferred so everything is done single-threaded on the main thread.

This is not really a solution that works for a release cause having the navigation mesh baking forced to single-thread will cause unavoidable and heavy performance issues for projects that change navigation meshes at runtime.

Steps to reproduce

Try to bake a navigation mesh at runtime with a background thread in a debug build.

Minimal reproduction project

see steps