[ssh] Bad exit status retrieval

**Describe the bug**
PlainSSHProcess attempts to retrieve the exit status by registering a callback and polling. The issue is that the callback is only registered for the duration of the call, so it will miss the return code if it does not come in that time window. The callbacks should ideally be registered for the lifetime of the channel.

Start by locating PlainSSHProcess and tracing how it registers the exit-status callback and polls for the result. Check the channel lifecycle and existing SSH-related tests, if present. Done means exit status retrieval remains reliable when the callback response arrives outside the current call window.

Written by the indexing model from the issue text.