Mark a conversation as read

Marks all unread incoming messages in the conversation as read.

For WhatsApp, this also sends read receipts (blue ticks) to the contact, EXCEPT on coexistence accounts (where the WhatsApp Business app on the customer's phone owns read state and we never override it).

This is the explicit, human-driven counterpart to GET .../messages, which is side-effect-free and does NOT mark anything read. Call this when a user actually views the conversation.