Re-resolve anchor before applying AI inline assist edits (#43103)

Closes #39088

Release Notes:

- Fixed AI assistant edits being scrambled when file was modified while
it was open

--

Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>

crates/agent_ui/src/buffer_codegen.rs

@@ -491,6 +491,14 @@ impl CodegenAlternative {
         cx: &mut Context<Self>,
     ) {
         let start_time = Instant::now();
+
+        // Make a new snapshot and re-resolve anchor in case the document was modified.
+        // This can happen often if the editor loses focus and is saved + reformatted,
+        // as in https://github.com/zed-industries/zed/issues/39088
+        self.snapshot = self.buffer.read(cx).snapshot(cx);
+        self.range = self.snapshot.anchor_after(self.range.start)
+            ..self.snapshot.anchor_after(self.range.end);
+
         let snapshot = self.snapshot.clone();
         let selected_text = snapshot
             .text_for_range(self.range.start..self.range.end)