When a user replies in a thread and @mentions an agent, the enqueued
task's trigger_comment_id was incorrectly set to the parent (thread
root) comment instead of the reply that contained the mention. This
caused the agent to read the wrong comment and miss the user's actual
instructions.
Always pass comment.ID to EnqueueTaskForMention so agents see the
comment that triggered them.
Fixes MUL-708