test(todo): add comprehensive unit tests for todo features

Add tests for:
- @Done insertion at various line positions
- Future task filtering with show_future flag
- Task sorting by moment ascending
- Error message formatting for all new error variants
- Trailing newline preservation

src/cli/commands/todo.rs

@@ -170,6 +170,26 @@ pub fn run() -> Result<(), StreamdError> {
 
 #[cfg(test)]
 mod tests {
+    use super::*;
+    use chrono::{Duration, TimeZone};
+    use indexmap::IndexMap;
+
+    fn make_task_shard(moment: chrono::DateTime<Utc>, file: &str) -> LocalizedShard {
+        let mut location = IndexMap::new();
+        location.insert("file".to_string(), file.to_string());
+        location.insert("task".to_string(), "open".to_string());
+
+        LocalizedShard {
+            markers: vec!["Task".to_string()],
+            tags: vec![],
+            start_line: 1,
+            end_line: 1,
+            moment,
+            location,
+            children: vec![],
+        }
+    }
+
     #[test]
     fn test_insert_done_after_task() {
         let line = "Some content @Task with more text";
@@ -184,6 +204,13 @@ mod tests {
         assert_eq!(result, "Some content @Task @Done");
     }
 
+    #[test]
+    fn test_insert_done_only_first_task() {
+        let line = "Some @Task content @Task again";
+        let result = line.replacen("@Task", "@Task @Done", 1);
+        assert_eq!(result, "Some @Task @Done content @Task again");
+    }
+
     #[test]
     fn test_task_count_single() {
         let line = "Some content @Task with more text";
@@ -201,4 +228,104 @@ mod tests {
         let line = "Some content without task marker";
         assert_eq!(line.matches("@Task").count(), 0);
     }
+
+    #[test]
+    fn test_filter_future_tasks_excludes_future_when_show_future_false() {
+        let now = Utc::now();
+        let past = now - Duration::hours(1);
+        let future = now + Duration::hours(1);
+
+        let tasks = vec![
+            make_task_shard(past, "past.md"),
+            make_task_shard(future, "future.md"),
+        ];
+
+        let filtered: Vec<_> = tasks
+            .into_iter()
+            .filter(|shard| shard.moment <= now)
+            .collect();
+
+        assert_eq!(filtered.len(), 1);
+        assert_eq!(filtered[0].location.get("file").unwrap(), "past.md");
+    }
+
+    #[test]
+    fn test_filter_future_tasks_includes_all_when_show_future_true() {
+        let now = Utc::now();
+        let past = now - Duration::hours(1);
+        let future = now + Duration::hours(1);
+
+        let tasks = vec![
+            make_task_shard(past, "past.md"),
+            make_task_shard(future, "future.md"),
+        ];
+
+        let filtered: Vec<_> = tasks.into_iter().filter(|_| true).collect();
+
+        assert_eq!(filtered.len(), 2);
+    }
+
+    #[test]
+    fn test_sort_tasks_by_moment_ascending() {
+        let oldest = Utc.with_ymd_and_hms(2020, 1, 1, 0, 0, 0).unwrap();
+        let middle = Utc.with_ymd_and_hms(2021, 1, 1, 0, 0, 0).unwrap();
+        let newest = Utc.with_ymd_and_hms(2022, 1, 1, 0, 0, 0).unwrap();
+
+        let mut tasks = vec![
+            make_task_shard(newest, "newest.md"),
+            make_task_shard(oldest, "oldest.md"),
+            make_task_shard(middle, "middle.md"),
+        ];
+
+        tasks.sort_by(|a, b| a.moment.cmp(&b.moment));
+
+        assert_eq!(tasks[0].location.get("file").unwrap(), "oldest.md");
+        assert_eq!(tasks[1].location.get("file").unwrap(), "middle.md");
+        assert_eq!(tasks[2].location.get("file").unwrap(), "newest.md");
+    }
+
+    #[test]
+    fn test_preserve_trailing_newline() {
+        let content_with_newline = "line1\nline2\n";
+        let content_without_newline = "line1\nline2";
+
+        assert!(content_with_newline.ends_with('\n'));
+        assert!(!content_without_newline.ends_with('\n'));
+    }
+
+    #[test]
+    fn test_invalid_task_number_zero() {
+        let result = StreamdError::InvalidTaskNumber(0, 5);
+        assert_eq!(
+            result.to_string(),
+            "Invalid task number 0: only 5 tasks available"
+        );
+    }
+
+    #[test]
+    fn test_invalid_task_number_exceeds_count() {
+        let result = StreamdError::InvalidTaskNumber(10, 3);
+        assert_eq!(
+            result.to_string(),
+            "Invalid task number 10: only 3 tasks available"
+        );
+    }
+
+    #[test]
+    fn test_multiple_task_markers_error_message() {
+        let result = StreamdError::MultipleTaskMarkers("/path/file.md".to_string(), 42);
+        assert_eq!(
+            result.to_string(),
+            "Multiple @Task markers found in /path/file.md:42 - cannot auto-insert @Done"
+        );
+    }
+
+    #[test]
+    fn test_no_task_marker_error_message() {
+        let result = StreamdError::NoTaskMarker("/path/file.md".to_string(), 42);
+        assert_eq!(
+            result.to_string(),
+            "No @Task marker found in /path/file.md:42"
+        );
+    }
 }