test: Unit-Tests für clean_filename() hinzugefügt

- 8 Tests für verschiedene Edge-Cases:
  - Basis-Funktionalität (Spaces, Klammern)
  - Versteckte Dateien (.gitignore, .my config)
  - Umlaute (Müller → Mueller)
  - Extensions (.tar.gz, mehrfache Punkte)
  - Spezial-Identifier (C++, C#)
  - Apostrophe (O'Reilly → OReilly)
  - Leere Dateinamen nach Bereinigung
  - Dateien die keine Änderung benötigen

Alle Tests bestehen ✓

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/sanitizer.rs

@@ -192,3 +192,169 @@ pub fn is_safe_rename(src: &Path, dst: &Path, force: bool) -> bool {
 
     true
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::ffi::OsStr;
+
+    fn make_test_config() -> Config {
+        let mut replacements = std::collections::HashMap::new();
+        replacements.insert("ä".to_string(), "ae".to_string());
+        replacements.insert("ö".to_string(), "oe".to_string());
+        replacements.insert("ü".to_string(), "ue".to_string());
+        replacements.insert("ß".to_string(), "ss".to_string());
+        Config { replacements }
+    }
+
+    #[test]
+    fn test_clean_filename_basic() {
+        let config = Config::default();
+
+        // Spaces should become underscores
+        assert_eq!(
+            clean_filename(OsStr::new("test file.txt"), &config, false),
+            Some("test_file.txt".to_string())
+        );
+
+        // Parentheses should become underscores
+        assert_eq!(
+            clean_filename(OsStr::new("file (1).txt"), &config, false),
+            Some("file_1.txt".to_string())
+        );
+
+        // Multiple underscores should be collapsed
+        assert_eq!(
+            clean_filename(OsStr::new("test__file.txt"), &config, false),
+            Some("test_file.txt".to_string())
+        );
+    }
+
+    #[test]
+    fn test_clean_filename_hidden_files() {
+        let config = Config::default();
+
+        // Hidden files should keep their leading dot
+        assert_eq!(
+            clean_filename(OsStr::new(".gitignore"), &config, false),
+            None // No change needed
+        );
+
+        // Hidden files with spaces
+        assert_eq!(
+            clean_filename(OsStr::new(".my config"), &config, false),
+            Some(".my_config".to_string())
+        );
+
+        // Hidden files with extension
+        assert_eq!(
+            clean_filename(OsStr::new(".test file.txt"), &config, false),
+            Some(".test_file.txt".to_string())
+        );
+
+        // Multiple leading dots
+        assert_eq!(
+            clean_filename(OsStr::new("...strange"), &config, false),
+            Some(".unnamed.strange".to_string())
+        );
+    }
+
+    #[test]
+    fn test_clean_filename_umlauts() {
+        let config = make_test_config();
+
+        // German umlauts
+        assert_eq!(
+            clean_filename(OsStr::new("Müller.pdf"), &config, false),
+            Some("Mueller.pdf".to_string())
+        );
+
+        assert_eq!(
+            clean_filename(OsStr::new("schön.txt"), &config, false),
+            Some("schoen.txt".to_string())
+        );
+
+        assert_eq!(
+            clean_filename(OsStr::new("Größe.doc"), &config, false),
+            Some("Groesse.doc".to_string())
+        );
+    }
+
+    #[test]
+    fn test_clean_filename_extensions() {
+        let config = Config::default();
+
+        // Single extension
+        assert_eq!(
+            clean_filename(OsStr::new("test file.txt"), &config, false),
+            Some("test_file.txt".to_string())
+        );
+
+        // Double extension (currently only keeps last)
+        assert_eq!(
+            clean_filename(OsStr::new("archive.tar.gz"), &config, false),
+            None // No special chars to clean
+        );
+
+        // Multiple dots
+        assert_eq!(
+            clean_filename(OsStr::new("foo..bar.txt"), &config, false),
+            Some("foo.bar.txt".to_string())
+        );
+    }
+
+    #[test]
+    fn test_clean_filename_special_identifiers() {
+        let config = Config::default();
+
+        // C++ should be preserved
+        assert_eq!(
+            clean_filename(OsStr::new("test C++.txt"), &config, false),
+            Some("test_C++.txt".to_string())
+        );
+
+        // C# should be preserved
+        assert_eq!(
+            clean_filename(OsStr::new("guide C#.pdf"), &config, false),
+            Some("guide_C#.pdf".to_string())
+        );
+    }
+
+    #[test]
+    fn test_clean_filename_no_change_needed() {
+        let config = Config::default();
+
+        // Already clean filenames should return None
+        assert_eq!(
+            clean_filename(OsStr::new("clean_file.txt"), &config, false),
+            None
+        );
+
+        assert_eq!(
+            clean_filename(OsStr::new("another-file.pdf"), &config, false),
+            None
+        );
+    }
+
+    #[test]
+    fn test_clean_filename_empty_after_cleaning() {
+        let config = Config::default();
+
+        // File with only special chars should become "unnamed"
+        assert_eq!(
+            clean_filename(OsStr::new("###.txt"), &config, false),
+            Some("unnamed.txt".to_string())
+        );
+    }
+
+    #[test]
+    fn test_clean_filename_apostrophe() {
+        let config = Config::default();
+
+        // Apostrophes should be removed (not replaced with underscore)
+        assert_eq!(
+            clean_filename(OsStr::new("O'Reilly.pdf"), &config, false),
+            Some("OReilly.pdf".to_string())
+        );
+    }
+}