test: add unit test for IconManager::supports_themes with empty manager (#29)

Add a unit test to verify that `IconManager::supports_themes` correctly returns `false`
for any icon name when the manager is empty (initialized via `IconManager::new()`).
This verifies the default behavior of the theme support mapping.

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>
Co-authored-by: bearice <270121+bearice@users.noreply.github.com>

Author: bearice

src/icon_manager.rs

@@ -251,3 +251,16 @@ impl Default for IconManager {
         Self::new()
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_supports_themes_empty_manager() {
+        let manager = IconManager::new();
+        assert!(!manager.supports_themes("cat"));
+        assert!(!manager.supports_themes("parrot"));
+        assert!(!manager.supports_themes(""));
+    }
+}