Mark CesiumForUnreal plugin Optional in project (deprecating project-level Cesium)

The CesiumForUnreal plugin was removed from Plugins/ on disk, but Ralpha.uproject
still listed it as Enabled with no "Optional" flag — a hard project requirement
UE can't satisfy, causing missing-modules/load instability (a likely contributor
to editor crashes this session). Marking it "Optional": true so a missing Cesium
no longer blocks project load. The engine-level Cesium install
(Engine/Plugins/Marketplace) still satisfies the reference when present, and the
C++ build already auto-disables Cesium (WITH_CESIUM=0) when the plugin is absent.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Ralpha.uproject

@@ -28,6 +28,10 @@
 			"Name": "MovieRenderPipeline",
 			"Enabled": true
 		},
+		{
+			"Name": "OpenColorIO",
+			"Enabled": true
+		},
 		{
 			"Name": "RuntimeMetaHumanLipSync",
 			"Enabled": false
@@ -55,7 +59,8 @@
 		},
 		{
 			"Name": "CesiumForUnreal",
-			"Enabled": true
+			"Enabled": true,
+			"Optional": true
 		},
 		{
 			"Name": "CameraCalibrationCore",
@@ -64,6 +69,26 @@
 		{
 			"Name": "CameraCalibration",
 			"Enabled": false
+		},
+		{
+			"Name": "Bridge",
+			"Enabled": true
+		},
+		{
+			"Name": "Fab",
+			"Enabled": true
+		},
+		{
+			"Name": "FabLauncher",
+			"Enabled": true
+		},
+		{
+			"Name": "MetaHumanSDK",
+			"Enabled": true
+		},
+		{
+			"Name": "MetaHumanCharacter",
+			"Enabled": true
 		}
 	]
-}
+}