ssh remoting: Fix SSH connection not being closed (#18329)

This fixes the `SshSession` being leaked.

There were two leaks:

1. `Arc<SshSession>` itself got leaked into the `SettingsObserver` that
   lives as long as the application. Fixed with a weak reference.
2. The two tasks spawned by an `SshSession` had a circular dependency
   and didn't exit while the other one was running. Fixed by fixing (1)
   and then attaching one of the tasks to the `SshSession`, which means
   it gets dropped with the session itself, which leads the other task
   to error and exit.

Co-authored-by: Bennet <bennet@zed.dev>

Release Notes:

- N/A

---------

Co-authored-by: Bennet <bennet@zed.dev>

crates/recent_projects/src/recent_projects.rs

@@ -509,7 +509,7 @@ impl PickerDelegate for RecentProjectsDelegate {
                                         .color(Color::Muted)
                                         .into_any_element()
                                 }
-                                SerializedWorkspaceLocation::Ssh(_) => Icon::new(IconName::Screen)
+                                SerializedWorkspaceLocation::Ssh(_) => Icon::new(IconName::Server)
                                     .color(Color::Muted)
                                     .into_any_element(),
                                 SerializedWorkspaceLocation::DevServer(_) => {