Harden render validation tests for input paths and empty transcript output

internal/config/config_test.go

@@ -895,6 +895,46 @@ func TestNewRenderConfigAppliesDefaultsAndFlags(t *testing.T) {
 	}
 }
 
+func TestNewRenderConfigRejectsMissingAndDirectoryInputFile(t *testing.T) {
+	dir := t.TempDir()
+	output := filepath.Join(dir, "rendered.md")
+
+	missingInput := filepath.Join(dir, "missing.json")
+	_, err := NewRenderConfig(validRenderOptions(missingInput, output))
+	if err == nil {
+		t.Fatal("expected missing input-file error")
+	}
+	if !strings.Contains(err.Error(), "--input-file") {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	inputDir := filepath.Join(dir, "input-dir")
+	if err := os.MkdirAll(inputDir, 0o700); err != nil {
+		t.Fatalf("mkdir input dir: %v", err)
+	}
+	_, err = NewRenderConfig(validRenderOptions(inputDir, output))
+	if err == nil {
+		t.Fatal("expected directory input-file error")
+	}
+	if !strings.Contains(err.Error(), "is a directory, not a file") {
+		t.Fatalf("unexpected error: %v", err)
+	}
+}
+
+func TestNewRenderConfigRejectsMissingOutputParent(t *testing.T) {
+	dir := t.TempDir()
+	input := writeTempFile(t, dir, "input.json")
+	output := filepath.Join(dir, "missing-parent", "rendered.md")
+
+	_, err := NewRenderConfig(validRenderOptions(input, output))
+	if err == nil {
+		t.Fatal("expected output parent directory error")
+	}
+	if !strings.Contains(err.Error(), "--output-file parent directory") {
+		t.Fatalf("unexpected error: %v", err)
+	}
+}
+
 func assertPositiveFloatEnvValidation(t *testing.T, envName string) {
 	t.Helper()