fix: use splitMessage utility, IDs from search, and parallel deletions in memory command

- Use splitMessage() utility instead of manual substring truncation for
  Discord's 2000-char limit, properly handling multi-byte characters (#1)
- Include memory IDs in searchMemories results and use them directly in
  handleForgetTopic instead of fragile text equality matching (#2)
- Parallelize deleteMemory calls with Promise.allSettled instead of
  sequential for loop (#3)
- Verify deferReply is called in all forget test variants (#7)

Author: BillChirico