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/rpc/src/proto_client.rs

@@ -10,11 +10,29 @@ use proto::{
     error::ErrorExt as _, AnyTypedEnvelope, EntityMessage, Envelope, EnvelopedMessage,
     RequestMessage, TypedEnvelope,
 };
-use std::{any::TypeId, sync::Arc};
+use std::{
+    any::TypeId,
+    sync::{Arc, Weak},
+};
 
 #[derive(Clone)]
 pub struct AnyProtoClient(Arc<dyn ProtoClient>);
 
+impl AnyProtoClient {
+    pub fn downgrade(&self) -> AnyWeakProtoClient {
+        AnyWeakProtoClient(Arc::downgrade(&self.0))
+    }
+}
+
+#[derive(Clone)]
+pub struct AnyWeakProtoClient(Weak<dyn ProtoClient>);
+
+impl AnyWeakProtoClient {
+    pub fn upgrade(&self) -> Option<AnyProtoClient> {
+        self.0.upgrade().map(AnyProtoClient)
+    }
+}
+
 pub trait ProtoClient: Send + Sync {
     fn request(
         &self,