feat: inject ContentsRepository into CommentsService for better integration

- Added `ContentsRepository` as a dependency to `CommentsService` and updated tests for mock setup.
- Adjusted import order in relevant files to align with project standards.

backend/src/comments/comments.module.ts

@@ -1,4 +1,4 @@
-import { Module, forwardRef } from "@nestjs/common";
+import { forwardRef, Module } from "@nestjs/common";
 import { AuthModule } from "../auth/auth.module";
 import { ContentsModule } from "../contents/contents.module";
 import { RealtimeModule } from "../realtime/realtime.module";