Description

Properly closes the doctor connections.

• active_side is the side that last receives data. Thus it needs to indicate it's happy via conn.close(...). For good measure, we also conn.closed().await.
• passive_side is the side that last sends data and does accept_bi. We race this loop with the conn.closed() because that indicates the other side is done. As this is not an error, we just exit.
• If the passive_side errors, we send a conn.close() earlier. This would be optional, if the Endpoint were shutdown gracefully... Which never happens. But that's another PR.

Change checklist

• Self-review.
• Documentation updates following the style guide, if relevant.
• [ ] Tests if relevant.
• All breaking changes documented.