refactor: improve error handling and documentation in PermissionSets

- Add explicit ArgumentError for invalid permission set names with helpful message
- Soften performance claim in documentation (intended to be constant-time)
- Add tests for error handling
- Improve maintainability with guard clause for invalid inputs

test/mv/authorization/permission_sets_test.exs

@@ -567,4 +567,27 @@ defmodule Mv.Authorization.PermissionSetsTest do
                {:error, :invalid_permission_set}
     end
   end
+
+  describe "get_permissions/1 - error handling" do
+    test "raises ArgumentError for invalid permission set with helpful message" do
+      assert_raise ArgumentError,
+                   ~r/invalid permission set: :invalid\. Must be one of:/,
+                   fn ->
+                     PermissionSets.get_permissions(:invalid)
+                   end
+    end
+
+    test "error message includes all valid permission sets" do
+      error =
+        assert_raise ArgumentError, fn ->
+          PermissionSets.get_permissions(:unknown)
+        end
+
+      error_message = Exception.message(error)
+      assert error_message =~ "own_data"
+      assert error_message =~ "read_only"
+      assert error_message =~ "normal_user"
+      assert error_message =~ "admin"
+    end
+  end
 end