Clean up MVP output handling

internal/modules/extract/dnd/spells/config_test.go

@@ -257,8 +257,9 @@ func (dndSpellsOutput) Key() string {
 
 func (dndSpellsOutput) Encode(ctx context.Context, req contracts.OutputRequest) (contracts.OutputResult, error) {
 	return contracts.OutputResult{
-		Bytes:       []byte(`{"encoded":true}`),
-		ContentType: "application/json",
+		Files: []contracts.OutputFile{
+			{Name: "output.json", ContentType: "application/json", Bytes: []byte(`{"encoded":true}`)},
+		},
 	}, nil
 }
 

internal/modules/extract/dnd/spells/runner_test.go

@@ -102,8 +102,11 @@ func TestRunnerProcessesSeriatimInputWithDNDSpellsExtractor(t *testing.T) {
 		extractorMetadata["response_schema_name"] != ResponseSchemaName {
 		t.Fatalf("extractor metadata = %#v, want prompt/schema identifiers", extractorMetadata)
 	}
-	if output.ContentType != "application/json" {
-		t.Fatalf("ContentType = %q, want application/json", output.ContentType)
+	if len(output.OutputFiles) != 1 {
+		t.Fatalf("len(OutputFiles) = %d, want 1", len(output.OutputFiles))
+	}
+	if output.OutputFiles[0].ContentType != "application/json" {
+		t.Fatalf("ContentType = %q, want application/json", output.OutputFiles[0].ContentType)
 	}
 }