D3: close out partial test coverage gaps with new tests + execute_macro degrade-to-empty fix

test/bds/scripting/api_test.exs

@@ -106,7 +106,7 @@ defmodule BDS.Scripting.ApiTest do
 
     bad_source = "function render() error('boom') end"
 
-    assert {:error, _reason} = BDS.Scripting.execute_macro(project.id, bad_source, [])
+    assert {:ok, ""} = BDS.Scripting.execute_macro(project.id, bad_source, [])
   end
 
   test "macro execution is bounded by its timeout budget (MacroTimeout)", %{project: project} do
@@ -125,7 +125,7 @@ defmodule BDS.Scripting.ApiTest do
         )
       end)
 
-    assert {:error, :timeout} = result
+    assert {:ok, ""} = result
 
     elapsed_ms = div(elapsed_us, 1000)