edit_prediction: Normalize prompt paths to Unix style (#61792)

# Objective

- Fix V4 edit prediction requests failing for local Windows projects
because native path separators do not match Unix-formatted context paths
on the server.

## Solution

- Canonicalize serialized buffer paths through the existing `RelPath`
representation.

## Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [ ] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

## Suggested .rules additions

- Paths serialized for edit prediction APIs must use `RelPath`'s Unix
representation rather than platform-native `PathBuf` formatting.

---

Release Notes:

- N/A

crates/edit_prediction/src/edit_prediction.rs

@@ -70,7 +70,7 @@ use std::sync::Arc;
 use std::time::{Duration, Instant};
 
 use thiserror::Error;
-use util::ResultExt as _;
+use util::{ResultExt as _, rel_path::RelPath};
 
 pub mod cursor_excerpt;
 pub mod data_collection;
@@ -904,11 +904,14 @@ pub(crate) fn buffer_path_with_id_fallback(
     snapshot: &TextBufferSnapshot,
     cx: &App,
 ) -> Arc<Path> {
-    if let Some(file) = file {
-        file.full_path(cx).into()
-    } else {
-        Path::new(&format!("untitled-{}", snapshot.remote_id())).into()
-    }
+    let Some(file) = file else {
+        return Path::new(&format!("untitled-{}", snapshot.remote_id())).into();
+    };
+    let full_path = file.full_path(cx);
+    let Some(path) = RelPath::new(&full_path, file.path_style(cx)).ok() else {
+        return Path::new(&format!("untitled-{}", snapshot.remote_id())).into();
+    };
+    path.as_std_path().into()
 }
 
 fn predict_edits_request_trigger_from_editor_trigger(

crates/edit_prediction/src/edit_prediction_tests.rs

@@ -4127,13 +4127,22 @@ async fn test_edit_prediction_settled_drops_future_events_when_their_oss_status_
 }
 
 #[gpui::test]
-fn test_buffer_path_with_id_fallback_for_untitled_buffers(cx: &mut TestAppContext) {
+fn test_buffer_path_with_id_fallback(cx: &mut TestAppContext) {
     let buffer_1 = cx.new(|cx| Buffer::local("one", cx));
     let buffer_2 = cx.new(|cx| Buffer::local("two", cx));
 
     let snapshot_1 = buffer_1.read_with(cx, |buffer, _| buffer.text_snapshot());
     let snapshot_2 = buffer_2.read_with(cx, |buffer, _| buffer.text_snapshot());
 
+    let windows_file: Arc<dyn language::File> = Arc::new(language::TestFile {
+        path: util::rel_path::rel_path("src/main.rs").into(),
+        root_name: "workspace".into(),
+        local_root: None,
+    });
+    let windows_path =
+        cx.read(|cx| buffer_path_with_id_fallback(Some(&windows_file), &snapshot_1, cx));
+    assert_eq!(windows_path.to_string_lossy(), "workspace/src/main.rs");
+
     let path_1 = cx.read(|cx| buffer_path_with_id_fallback(None, &snapshot_1, cx));
     let path_2 = cx.read(|cx| buffer_path_with_id_fallback(None, &snapshot_2, cx));