AuthN: Introduce DefaultOrgID function for managed service accounts (#93432)

* Managed Service Accounts: Use AutoAssignOrgID

* Fix the IsExternalServiceAccount function

* Reassign service account role

* Account for AutoAssignOrg

* Update pkg/services/serviceaccounts/models.go

* Simplify IsExternalServiceAccount function

* Add tests

* Easier to understand test

* Revert small change

pkg/services/accesscontrol/database/externalservices.go

@@ -240,6 +240,15 @@ func (*AccessControlStore) saveUserAssignment(ctx context.Context, sess *db.Sess
 		return errGetAssigns
 	}
 
+	// Revoke assignment if it's assigned to another user or service account
+	if len(assignments) > 0 && assignments[0].UserID != assignment.UserID {
+		if _, errDel := sess.Where("role_id = ?", assignment.RoleID).Delete(&accesscontrol.UserRole{}); errDel != nil {
+			return errDel
+		}
+		assignments = nil
+	}
+
+	// If no assignment exists, insert a new one.
 	if len(assignments) == 0 {
 		if _, errInsert := sess.Insert(&assignment); errInsert != nil {
 			return errInsert
@@ -247,11 +256,6 @@ func (*AccessControlStore) saveUserAssignment(ctx context.Context, sess *db.Sess
 		return nil
 	}
 
-	// Ensure the role was assigned only to this service account
-	if len(assignments) > 1 || assignments[0].UserID != assignment.UserID {
-		return errors.New("external service role assigned to another user or service account")
-	}
-
 	// Ensure the assignment is in the correct organization
 	_, errUpdate := sess.Where("role_id = ? AND user_id = ?", assignment.RoleID, assignment.UserID).MustCols("org_id").Update(&assignment)
 	return errUpdate