NavigationServer: Restore constness for thread safe `get_singleton`

This was removed by mistake in #47024, NavigationServer uses internal
mutability for thread safety, and removing `const` breaks the contract.

scene/2d/navigation_region_2d.cpp

@@ -363,10 +363,10 @@ void NavigationRegion2D::set_enabled(bool p_enabled) {
 
 	if (!enabled) {
 		NavigationServer2D::get_singleton()->region_set_map(region, RID());
-		NavigationServer2D::get_singleton()->disconnect("map_changed", callable_mp(this, &NavigationRegion2D::_map_changed));
+		NavigationServer2D::get_singleton_mut()->disconnect("map_changed", callable_mp(this, &NavigationRegion2D::_map_changed));
 	} else {
 		NavigationServer2D::get_singleton()->region_set_map(region, get_world_2d()->get_navigation_map());
-		NavigationServer2D::get_singleton()->connect("map_changed", callable_mp(this, &NavigationRegion2D::_map_changed));
+		NavigationServer2D::get_singleton_mut()->connect("map_changed", callable_mp(this, &NavigationRegion2D::_map_changed));
 	}
 
 	if (Engine::get_singleton()->is_editor_hint() || get_tree()->is_debugging_navigation_hint()) {
@@ -402,7 +402,7 @@ void NavigationRegion2D::_notification(int p_what) {
 		case NOTIFICATION_ENTER_TREE: {
 			if (enabled) {
 				NavigationServer2D::get_singleton()->region_set_map(region, get_world_2d()->get_navigation_map());
-				NavigationServer2D::get_singleton()->connect("map_changed", callable_mp(this, &NavigationRegion2D::_map_changed));
+				NavigationServer2D::get_singleton_mut()->connect("map_changed", callable_mp(this, &NavigationRegion2D::_map_changed));
 			}
 		} break;
 		case NOTIFICATION_TRANSFORM_CHANGED: {
@@ -411,7 +411,7 @@ void NavigationRegion2D::_notification(int p_what) {
 		case NOTIFICATION_EXIT_TREE: {
 			NavigationServer2D::get_singleton()->region_set_map(region, RID());
 			if (enabled) {
-				NavigationServer2D::get_singleton()->disconnect("map_changed", callable_mp(this, &NavigationRegion2D::_map_changed));
+				NavigationServer2D::get_singleton_mut()->disconnect("map_changed", callable_mp(this, &NavigationRegion2D::_map_changed));
 			}
 		} break;
 		case NOTIFICATION_DRAW: {

servers/navigation_server_2d.cpp

@@ -204,7 +204,7 @@ void NavigationServer2D::_bind_methods() {
 NavigationServer2D::NavigationServer2D() {
 	singleton = this;
 	ERR_FAIL_COND_MSG(!NavigationServer3D::get_singleton(), "The Navigation3D singleton should be initialized before the 2D one.");
-	NavigationServer3D::get_singleton()->connect("map_changed", callable_mp(this, &NavigationServer2D::_emit_map_changed));
+	NavigationServer3D::get_singleton_mut()->connect("map_changed", callable_mp(this, &NavigationServer2D::_emit_map_changed));
 }
 
 NavigationServer2D::~NavigationServer2D() {

servers/navigation_server_2d.h

@@ -52,7 +52,7 @@ protected:
 
 public:
 	/// Thread safe, can be used across many threads.
-	static NavigationServer2D *get_singleton() { return singleton; }
+	static const NavigationServer2D *get_singleton() { return singleton; }
 
 	/// MUST be used in single thread!
 	static NavigationServer2D *get_singleton_mut() { return singleton; }

servers/navigation_server_3d.cpp

@@ -84,7 +84,7 @@ void NavigationServer3D::_bind_methods() {
 	ADD_SIGNAL(MethodInfo("map_changed", PropertyInfo(Variant::RID, "map")));
 }
 
-NavigationServer3D *NavigationServer3D::get_singleton() {
+const NavigationServer3D *NavigationServer3D::get_singleton() {
 	return singleton;
 }
 

servers/navigation_server_3d.h

@@ -55,7 +55,7 @@ protected:
 
 public:
 	/// Thread safe, can be used across many threads.
-	static NavigationServer3D *get_singleton();
+	static const NavigationServer3D *get_singleton();
 
 	/// MUST be used in single thread!
 	static NavigationServer3D *get_singleton_mut();