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/worktree/src/worktree.rs

@@ -472,7 +472,7 @@ impl Worktree {
                 disconnected: false,
             };
 
-            // Apply updates to a separate snapshto in a background task, then
+            // Apply updates to a separate snapshot in a background task, then
             // send them to a foreground task which updates the model.
             cx.background_executor()
                 .spawn(async move {