[lib] Fix creating thick threads on added members robotext

Summary:
This diff fixes creating thick threads on added members robotext by assigning the same message id to the two types of messages sent to users.

https://linear.app/comm/issue/ENG-9574/creating-thick-threads-on-robotexts-is-not-working

Test Plan:

1. User A creates a local dm
2. User A adds user B
3. User A creates a thread on "you added user B" robotext
4. Verify that user B sees the thread

Reviewers: kamil, tomek, ashoat

Reviewed By: ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13758