Third-Party Conversations: support multiple recipients and fix reply attribution

Ran into two related snags with Third-Party Conversations this week that I think are worth fixing.

1. Only one recipient allowed. You can only address a Third-Party Conversation to one person. If a vendor issue needs more than one contact (like a primary tech and their backup, or someone from sales plus support), there’s no way to add them both without starting a whole separate thread for each person.

2. Replies from a CC’d colleague get misattributed. I had a Third-Party Conversation going with a vendor contact (“Mark”). When Mark looped in a coworker (“Theodore”) who replied on the thread, Gorelo showed Theodore’s message as if it came from Mark. Then when I hit reply, it only went back to Mark, so I had to start a brand new Third-Party Conversation just to write to Theodore directly.

Together these make it harder than it needs to be to handle any vendor conversation that involves more than one person on their end, which is honestly pretty common. Would love to see Third-Party Conversations support multiple To/Cc recipients (similar to how a normal email thread works), with replies correctly attributed to whoever actually sent them.