chore: Fix some violations of 'needless_pass_by_ref_mut' lint (#18795)

While this lint is allow-by-default, it seems pretty useful to get rid
of mutable borrows when they're not needed.

Closes #ISSUE

Release Notes:

- N/A

crates/client/src/client.rs

@@ -394,7 +394,7 @@ pub struct PendingEntitySubscription<T: 'static> {
 }
 
 impl<T: 'static> PendingEntitySubscription<T> {
-    pub fn set_model(mut self, model: &Model<T>, cx: &mut AsyncAppContext) -> Subscription {
+    pub fn set_model(mut self, model: &Model<T>, cx: &AsyncAppContext) -> Subscription {
         self.consumed = true;
         let mut handlers = self.client.handler_set.lock();
         let id = (TypeId::of::<T>(), self.remote_id);