<!-- PR description-->
* Prerequisite to the currently failing PR https://github.com/alcionai/corso/pull/5154
* Conversations tombstone IDs are currently 2 part ( `convID/threadID`).
* However, tombstones are looked up by `convID` part only, see [code](9a603d1f21/src/internal/m365/collection/groups/backup.go (L128)).
* Adding a change so that tombstone IDs are reduced to `convID`. Otherwise, we'd never delete tombstones while processing collections, and we'll run into `conflict: tombstone exists for a live collection` errors.
* This is safe to do as there is always a 1:1 relationship between `convID` and `threadID`. For e.g. attempting to create another thread inside a conversation creates a new conversation.
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#### Does this PR need a docs update or release note?
- [ ] ✅ Yes, it's included
- [ ] 🕐 Yes, but in a later PR
- [x] ⛔ No
#### Type of change
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- [ ] 🌻 Feature
- [x] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Supportability/Tests
- [ ] 💻 CI/Deployment
- [ ] 🧹 Tech Debt/Cleanup
#### Issue(s)
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* #<issue>
#### Test Plan
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- [x] 💪 Manual
- [x] ⚡ Unit test
- [ ] 💚 E2E
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