windows: Fix inconsistent separators in buffer headers and breadcrumbs (#38898)

Make `resolve_full_path` use the appropriate separators, and return a
`String`.

As part of fixing the fallout from that type change, this also fixes a
bunch of places in the agent code that were using `std::path::Path`
operations on paths that could be non-local, by changing them to operate
instead on strings and use the project's `PathStyle`.

This clears the way a bit for making `full_path` also return a string
instead of a `PathBuf`, but I've left that for a follow-up.

Release Notes:

- N/A

crates/editor/src/element.rs

@@ -3782,13 +3782,17 @@ impl EditorElement {
         let file = for_excerpt.buffer.file();
         let can_open_excerpts = Editor::can_open_excerpts_in_file(file);
         let path_style = file.map(|file| file.path_style(cx));
-        let relative_path = for_excerpt.buffer.resolve_file_path(cx, include_root);
-        let filename = relative_path
-            .as_ref()
-            .and_then(|path| Some(path.file_name()?.to_string_lossy().to_string()));
-        let parent_path = relative_path.as_ref().and_then(|path| {
-            Some(path.parent()?.to_string_lossy().to_string() + path_style?.separator())
-        });
+        let relative_path = for_excerpt.buffer.resolve_file_path(include_root, cx);
+        let (parent_path, filename) = if let Some(path) = &relative_path {
+            if let Some(path_style) = path_style {
+                let (dir, file_name) = path_style.split(path);
+                (dir.map(|dir| dir.to_owned()), Some(file_name.to_owned()))
+            } else {
+                (None, Some(path.clone()))
+            }
+        } else {
+            (None, None)
+        };
         let focus_handle = editor.focus_handle(cx);
         let colors = cx.theme().colors();
 

crates/editor/src/items.rs

@@ -963,13 +963,12 @@ impl Item for Editor {
             buffer
                 .snapshot()
                 .resolve_file_path(
-                    cx,
                     self.project
                         .as_ref()
                         .map(|project| project.read(cx).visible_worktrees(cx).count() > 1)
                         .unwrap_or_default(),
+                    cx,
                 )
-                .map(|path| path.to_string_lossy().to_string())
                 .unwrap_or_else(|| {
                     if multibuffer.is_singleton() {
                         multibuffer.title(cx).to_string()