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/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()