Fix panic in UnwrapSyntaxNode (#39139)

Closes #39139
Fixes ZED-1HY

Release Notes:

- Fixed a panic in UnwrapSyntaxNode in multi-buffers

crates/multi_buffer/src/multi_buffer.rs

@@ -79,12 +79,6 @@ pub struct MultiBuffer {
     buffer_changed_since_sync: Rc<Cell<bool>>,
 }
 
-#[derive(Clone, Debug, PartialEq, Eq)]
-pub enum MultiOrSingleBufferOffsetRange {
-    Single(Range<usize>),
-    Multi(Range<usize>),
-}
-
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub enum Event {
     ExcerptsAdded {
@@ -6077,19 +6071,17 @@ impl MultiBufferSnapshot {
     pub fn syntax_ancestor<T: ToOffset>(
         &self,
         range: Range<T>,
-    ) -> Option<(tree_sitter::Node<'_>, MultiOrSingleBufferOffsetRange)> {
+    ) -> Option<(tree_sitter::Node<'_>, Range<usize>)> {
         let range = range.start.to_offset(self)..range.end.to_offset(self);
         let mut excerpt = self.excerpt_containing(range.clone())?;
         let node = excerpt
             .buffer()
             .syntax_ancestor(excerpt.map_range_to_buffer(range))?;
         let node_range = node.byte_range();
-        let range = if excerpt.contains_buffer_range(node_range.clone()) {
-            MultiOrSingleBufferOffsetRange::Multi(excerpt.map_range_from_buffer(node_range))
-        } else {
-            MultiOrSingleBufferOffsetRange::Single(node_range)
+        if !excerpt.contains_buffer_range(node_range.clone()) {
+            return None;
         };
-        Some((node, range))
+        Some((node, excerpt.map_range_from_buffer(node_range)))
     }
 
     pub fn syntax_next_sibling<T: ToOffset>(