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/test/platform.rs

@@ -336,6 +336,7 @@ impl Platform for TestPlatform {
     fn prompt_for_new_path(
         &self,
         directory: &std::path::Path,
+        _suggested_name: Option<&str>,
     ) -> oneshot::Receiver<Result<Option<std::path::PathBuf>>> {
         let (tx, rx) = oneshot::channel();
         self.background_executor()