copilot: Fix onboarding into Copilot requires Zed restart (#26330)

Closes #25594

This PR fixes an issue where signing into Copilot required restarting
Zed.

Copilot depends on an OAuth token that comes from either `hosts.json` or
`apps.json`. Initially, both files don't exist. If neither file is
found, we fallback to watching `hosts.json` for updates. However, if the
auth process creates `apps.json`, we won't receive updates from it,
causing the UI to remain outdated.

This PR fixes that by watching the parent `github-copilot` directory
instead, which will always contain one of those files along with an
additional version file.

I have tested this on macOS and Linux Wayland.

Release Notes:

- Fixed an issue where signing into Copilot required restarting Zed.

crates/settings/src/settings_file.rs

@@ -1,5 +1,6 @@
 use crate::{settings_store::SettingsStore, Settings};
-use fs::Fs;
+use collections::HashSet;
+use fs::{Fs, PathEventKind};
 use futures::{channel::mpsc, StreamExt};
 use gpui::{App, BackgroundExecutor, ReadGlobal};
 use std::{path::PathBuf, sync::Arc, time::Duration};
@@ -78,6 +79,55 @@ pub fn watch_config_file(
     rx
 }
 
+pub fn watch_config_dir(
+    executor: &BackgroundExecutor,
+    fs: Arc<dyn Fs>,
+    dir_path: PathBuf,
+    config_paths: HashSet<PathBuf>,
+) -> mpsc::UnboundedReceiver<String> {
+    let (tx, rx) = mpsc::unbounded();
+    executor
+        .spawn(async move {
+            for file_path in &config_paths {
+                if fs.metadata(file_path).await.is_ok_and(|v| v.is_some()) {
+                    if let Ok(contents) = fs.load(file_path).await {
+                        if tx.unbounded_send(contents).is_err() {
+                            return;
+                        }
+                    }
+                }
+            }
+
+            let (events, _) = fs.watch(&dir_path, Duration::from_millis(100)).await;
+            futures::pin_mut!(events);
+
+            while let Some(event_batch) = events.next().await {
+                for event in event_batch {
+                    if config_paths.contains(&event.path) {
+                        match event.kind {
+                            Some(PathEventKind::Removed) => {
+                                if tx.unbounded_send(String::new()).is_err() {
+                                    return;
+                                }
+                            }
+                            Some(PathEventKind::Created) | Some(PathEventKind::Changed) => {
+                                if let Ok(contents) = fs.load(&event.path).await {
+                                    if tx.unbounded_send(contents).is_err() {
+                                        return;
+                                    }
+                                }
+                            }
+                            _ => {}
+                        }
+                    }
+                }
+            }
+        })
+        .detach();
+
+    rx
+}
+
 pub fn update_settings_file<T: Settings>(
     fs: Arc<dyn Fs>,
     cx: &App,