Add test for WriteToTempFile failure scenario

This new test verifies error handling for WriteToTempFile when the TMPDIR environment variable is set to an invalid directory. It ensures that the method fails as expected under these conditions, improving code robustness.

msg_nowin_test.go

@@ -61,3 +61,18 @@ func TestMsg_WriteToSendmail(t *testing.T) {
 		t.Errorf("WriteToSendmail failed: %s", err)
 	}
 }
+
+func TestMsg_WriteToTempFileFailed(t *testing.T) {
+	m := NewMsg()
+	_ = m.From("Toni Tester <tester@example.com>")
+	_ = m.To("Ellenor Tester <ellinor@example.com>")
+	m.SetBodyString(TypeTextPlain, "This is a test")
+
+	if err := os.Setenv("TMPDIR", "/invalid/directory/that/does/not/exist"); err != nil {
+		t.Errorf("failed to set TMPDIR environment variable: %s", err)
+	}
+	_, err := m.WriteToTempFile()
+	if err == nil {
+		t.Errorf("WriteToTempFile() did not fail as expected")
+	}
+}