test: snapshots of kcolorscheme and qpalette

AI disclosure: I asked GitHub Copilot (Claude Haiku 4.5) "What's the best way to add tests for my recently merged qt theming contributions?" It suggested the insta crate for golden testing the output strings as well as some unit tests. I implemented it myself.

cosmic-theme/src/output/qt56ct_output.rs

@@ -389,3 +389,27 @@ fn to_argb_hex(c: Srgba) -> String {
         c_u8.alpha, c_u8.red, c_u8.green, c_u8.blue
     )
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_color_to_argb_hex() {
+        let color = Srgba::new(0x33, 0x55, 0x77, 0xff);
+        let argb = to_argb_hex(color.into());
+        assert_eq!(argb, "#ff335577");
+    }
+
+    #[test]
+    fn test_light_default_qpalette() {
+        let light_default_qpalette = Theme::light_default().as_qpalette();
+        insta::assert_snapshot!(light_default_qpalette);
+    }
+
+    #[test]
+    fn test_dark_default_qpalette() {
+        let dark_default_qpalette = Theme::dark_default().as_qpalette();
+        insta::assert_snapshot!(dark_default_qpalette);
+    }
+}