fix: handle MySQL backend failures in comment API

[Alam-2U]

Summary

This PR fixes a regression in the MySQL forum backend where comments automatically deleted during spam moderation could trigger a 500 error during comment creation.

Root Cause

When a comment is created, moderate_and_flag_spam() may automatically soft-delete it if spam auto-deletion is enabled. In that case, the moderation result contains "soft_deleted" in actions_taken.

Both create_parent_comment() and create_child_comment() subsequently re-fetch the comment from the backend before preparing the API response. Since the comment has already been soft-deleted, the second fetch returns None, which is then passed to prepare_comment_api_response(), resulting in a 500 error.

The MongoDB implementation already handled this scenario by capturing the moderation result and avoiding the second fetch when the comment had been soft-deleted. The MySQL implementation missed this behavior.

Changes

• Preserve the original comment snapshot when the post-moderation lookup returns None.
• Fall back to the previously fetched comment instead of overwriting it with None.
• Prevent 500 errors for comments that are auto-deleted during spam moderation.

Ticket : LP-831