Suggest unsaved buffer content text as the default filename (#35707)

Closes #24672

This PR complements a feature added earlier by @JosephTLyons (in
https://github.com/zed-industries/zed/pull/32353) where the text is
considered as the tab title in a new buffer. It piggybacks off that
change and sets the title as the suggested filename in the save dialog
(completely mirroring the same functionality in VSCode):

![2025-08-05 11 50
28](https://github.com/user-attachments/assets/49ad9e4a-5559-44b0-a4b0-ae19890e478e)

Release Notes:

- Text entered in a new untitled buffer is considered as the default
filename when saving

crates/gpui/src/platform/mac/platform.rs

@@ -737,8 +737,13 @@ impl Platform for MacPlatform {
         done_rx
     }
 
-    fn prompt_for_new_path(&self, directory: &Path) -> oneshot::Receiver<Result<Option<PathBuf>>> {
+    fn prompt_for_new_path(
+        &self,
+        directory: &Path,
+        suggested_name: Option<&str>,
+    ) -> oneshot::Receiver<Result<Option<PathBuf>>> {
         let directory = directory.to_owned();
+        let suggested_name = suggested_name.map(|s| s.to_owned());
         let (done_tx, done_rx) = oneshot::channel();
         self.foreground_executor()
             .spawn(async move {
@@ -748,6 +753,11 @@ impl Platform for MacPlatform {
                     let url = NSURL::fileURLWithPath_isDirectory_(nil, path, true.to_objc());
                     panel.setDirectoryURL(url);
 
+                    if let Some(suggested_name) = suggested_name {
+                        let name_string = ns_string(&suggested_name);
+                        let _: () = msg_send![panel, setNameFieldStringValue: name_string];
+                    }
+
                     let done_tx = Cell::new(Some(done_tx));
                     let block = ConcreteBlock::new(move |response: NSModalResponse| {
                         let mut result = None;