Keep Teams' client id when a sent message is echoed

[itz4blitz]

Summary

• macOS CI on main failed twice in sending_editing_reacting_deleting_and_reading_go_to_teams: Teams had already been asked to post, but the message never looked accepted.
• The HTTP answer marks the row sent with the shell's client id. About 100ms later the echo arrives without clientmessageid copied onto the message, and the store replaces the row, dropping that id. A slow poll misses the window and waits out the 20s timeout.
• The echo now keeps clientmessageid (text or number) on our own messages, which is what the store uses to replace the pending row.

Test plan

• cargo test -p chatd --lib a_copy_of_our_message_keeps_the_client_id_teams_echoes
• cargo test -p chatd --test teams sending_editing_reacting_deleting_and_reading_go_to_teams
• CI on macOS, Linux, and Windows

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