feat: Implement add_tags function with tag merging and deduplication logic

src/tag_engine.rs

@@ -67,6 +67,18 @@ pub fn parse_tags(filename: &str) -> Result<(String, Vec<String>, String), Parse
     Ok((base_name, tags, extension))
 }
 
+pub fn add_tags(current: Vec<String>, new: Vec<String>) -> Vec<String> {
+    let mut result = current;
+    
+    for tag in new {
+        if !result.contains(&tag) {
+            result.push(tag);
+        }
+    }
+    
+    result
+}
+
 pub fn serialize_tags(base: &str, tags: &[String], extension: &str) -> String {
     let mut sorted_tags = tags.to_vec();
     sorted_tags.sort();
@@ -117,6 +129,46 @@ mod tests {
         assert_eq!(result, "README -- note");
     }
 
+    #[test]
+    fn test_add_tags_no_duplicates() {
+        let current = vec!["tag1".to_string(), "tag2".to_string()];
+        let new = vec!["tag3".to_string(), "tag4".to_string()];
+        let result = add_tags(current, new);
+        assert_eq!(result, vec!["tag1", "tag2", "tag3", "tag4"]);
+    }
+
+    #[test]
+    fn test_add_tags_with_duplicates() {
+        let current = vec!["tag1".to_string(), "tag2".to_string()];
+        let new = vec!["tag2".to_string(), "tag3".to_string()];
+        let result = add_tags(current, new);
+        assert_eq!(result, vec!["tag1", "tag2", "tag3"]);
+    }
+
+    #[test]
+    fn test_add_tags_case_sensitive() {
+        let current = vec!["Tag1".to_string(), "tag2".to_string()];
+        let new = vec!["tag1".to_string(), "Tag2".to_string()];
+        let result = add_tags(current, new);
+        assert_eq!(result, vec!["Tag1", "tag2", "tag1", "Tag2"]);
+    }
+
+    #[test]
+    fn test_add_tags_empty_current() {
+        let current = Vec::new();
+        let new = vec!["tag1".to_string(), "tag2".to_string()];
+        let result = add_tags(current, new);
+        assert_eq!(result, vec!["tag1", "tag2"]);
+    }
+
+    #[test]
+    fn test_add_tags_empty_new() {
+        let current = vec!["tag1".to_string(), "tag2".to_string()];
+        let new = Vec::new();
+        let result = add_tags(current, new);
+        assert_eq!(result, vec!["tag1", "tag2"]);
+    }
+
     #[test]
     fn test_validate_tag_valid() {
         assert!(validate_tag("valid-tag").is_ok());