Tighten prepared execution credential handling

internal/usecase/runner_test.go

@@ -1757,7 +1757,7 @@ func TestRunnerRunDirectAPIKeyBypassesMissingEnvAndReachesLLM(t *testing.T) {
 	llmClient := &fakeLLM{resp: &domain.GenerateResponse{Content: "ok"}}
 	runner := NewRunner(promptRepo, execRepo, nil, defaultArtifactReader(), defaultRenderer(), llmClient, nil, nil)
 
-	_, err := runner.Run(context.Background(), domain.RunRequest{
+	result, err := runner.Run(context.Background(), domain.RunRequest{
 		PromptID:  "p",
 		ProfileID: "exec",
 		APIKey:    directKey,
@@ -1769,6 +1769,9 @@ func TestRunnerRunDirectAPIKeyBypassesMissingEnvAndReachesLLM(t *testing.T) {
 	if llmClient.lastReq.Target.APIKey != directKey {
 		t.Fatalf("expected direct API key to reach LLM request")
 	}
+	if result.EffectiveModelParams.APIKey != "" {
+		t.Fatal("run result retained direct API key")
+	}
 	if llmClient.lastReq.Target.APIKeyEnv != "PROMPTKIT_MISSING_KEY" {
 		t.Fatalf("expected api_key_env name to remain on target, got %q", llmClient.lastReq.Target.APIKeyEnv)
 	}