Make the listCustomizations method caller force to be in BGT (#3958)

The caller (Black Caiman) is calling this method from EDT,
where an API call could happen that needs to be in BGT.
Enforce this on CW side while fixing this on BC side.

Author: andrewyuq

jetbrains-core/src/software/aws/toolkits/jetbrains/services/codewhisperer/customization/CodeWhispererModelConfigurator.kt

@@ -15,6 +15,7 @@ import com.intellij.openapi.components.State
 import com.intellij.openapi.components.Storage
 import com.intellij.openapi.components.service
 import com.intellij.openapi.project.Project
+import com.intellij.util.concurrency.annotations.RequiresBackgroundThread
 import com.intellij.util.xmlb.annotations.MapAnnotation
 import com.intellij.util.xmlb.annotations.Property
 import software.amazon.awssdk.services.codewhispererruntime.model.CodeWhispererRuntimeException
@@ -113,6 +114,7 @@ class DefaultCodeWhispererModelConfigurator : CodeWhispererModelConfigurator, Pe
         }
     }
 
+    @RequiresBackgroundThread
     override fun listCustomizations(project: Project, passive: Boolean): List<CustomizationUiItem>? =
         calculateIfIamIdentityCenterConnection(project) {
             // 1. invoke API and get result