Fix: Persist Default Reaction When Toggling from a Non-Default Reaction

Fix: Persist Default Reaction When Toggling from a Non-Default Reaction: a task in bird-critic-1.5-full-300 (Harbor dataset). The DiscourseReactions::ReactionManager service has a bug in its toggle! method: when a user has already reacted with a non-default reaction (e.g., "heart") and then…

The task

The `DiscourseReactions::ReactionManager` service has a bug in its `toggle!` method: when a user has already reacted with a non-default reaction (e.g., "heart") and then toggles to the default reaction (e.g., "+1"), the default reaction is silently dropped. The old reaction is removed correctly, but the new default…

Part of kochsnow/bird-critic-1.5-full-300.