Add "Fix with Assistant" code action on lines with diagnostics (#18163)

Release Notes:

- Added a new "Fix with Assistant" action on code with errors or
warnings.

---------

Co-authored-by: Nathan <nathan@zed.dev>

crates/project/src/project.rs

@@ -3247,7 +3247,7 @@ impl Project {
         buffer_handle: &Model<Buffer>,
         range: Range<T>,
         cx: &mut ModelContext<Self>,
-    ) -> Task<Vec<CodeAction>> {
+    ) -> Task<Result<Vec<CodeAction>>> {
         let buffer = buffer_handle.read(cx);
         let range = buffer.anchor_before(range.start)..buffer.anchor_before(range.end);
         self.lsp_store.update(cx, |lsp_store, cx| {