Innitial implementation of nested comments/replies

[RomanKholod]

No description provided.

[Serpantyn]

Closing this PR. This feature has been actively in progress under task #109 and assigned to me for the entire week. The complete implementation is already submitted in PR #121
You should have verified the board statuses and synced with the team before starting new work.

[sonarqubecloud]

Quality Gate failed

Failed conditions
0.0% Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube Cloud

[DrFaust555]

In practice, this PR should be closed as an outdated duplicate, and not refined.

Notes

1. The functionality is already implemented in merged PR #121.
   PR #121 contains replies, moderation, migration, full tree construction and broader tests. In the comment to #125, it was already requested to be closed due to duplication. Now the PR conflicts with dev.
2. There is no migration for ParentCommentId.
   The field is added to Comment, but the migration and model snapshot are not updated. The column and self-FK will not appear in the existing DB. The corresponding migration is present in #121.
3. Reply can be tied to a comment of another Streetcode.
   The Handler only checks the existence of parent, but not parentComment.StreetcodeId == command.Comment.StreetcodeId (code). As a result, the reply will be displayed in the wrong thread or will be lost altogether.
4. GET actually supports only one level of nesting.
   The request uses only .Include(c => c.Replies) and does not load the next levels (handler). At the same time, the API allows you to respond to a reply, so the third and subsequent levels are created, but not returned.
5. The author of the reply can be returned as Unknown User.
   There is no ThenInclude(r => r.User) for Replies, and the mapper substitutes Unknown User if User is missing. Since the repository uses AsNoTracking, automatic relationship fix-up will not fix this.
6. The tests do not pass.
   Result: 318 passed, 1 failed. A new Handle_ReturnsSuccess_WhenReplyIsValid is thrown: ParentCommentId == 1 is expected, actually null (assert).