fix: inconsistent loading of discussion thread response

[naincy128]

Description

Fixes an issue where inline replies within discussion threads could load inconsistently, causing some replies to appear or disappear between page refreshes.

Root Cause

Response comments were not being retrieved using backend-level pagination. Combined with frontend request race conditions and stale state handling, this could result in inconsistent loading of inline replies, especially in threads with many responses.

As a result, users could see different combinations of replies on successive page refreshes, even though the underlying discussion data was unchanged.

Changes

Backend (edx-platform)

• Updated response comment retrieval to use forum backend pagination APIs (get_comments and get_comments_count).
• Moved response-comment pagination to the forum backend for more reliable and efficient loading.
• Removed unused code related to the previous pagination approach.

Frontend (frontend-app-discussions)

• Added request cancellation using AbortController to prevent stale requests from overwriting newer data.
• Cleared stale child-comment state when reloading response comments.

Linked PR

frontend-app-discussions

Ticket

LP-529

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