Revisit the need for MainActor for DefaultMessages

[umair-ably]

Whilst I don't think this is wrong per-say, I'm not confident it is necessary.

My rationale here... given DefaultMessages is very close to the UI layer e.g. getting messages, it's not "wrong" for MainActor to be required here, however that's not why this was done.

I got an error message - Incorrect actor executor assumption of which there doesn't seem to be much resources on the internet about. This wasn't an issue prior to Swift 6 so it seems relatively new. Trial and error lead me to trying @MainActor and this worked.

Equally, I require subscriptionPoints to be mutable - without @MainActor, I get Stored property 'subscriptionPoints' of 'Sendable'-conforming class 'DefaultMessages' is mutable.

Creating this issue as a investigation task to revisit once we have:

1. More functionality implemented as part of chat-swift
2. More knowledge of Swift Concurrency

┆Issue is synchronized with this Jira Task by Unito