multi_buffer: Fix editor::ExpandExcerpts failing when cursor is at excerpt start (#42324)

The bug is easily verified by:

1. open any multi-buffer
2. place the cursor at the beginning of an excerpt
3. run the editor::ExpandExcerpts / editor: expand excerpts action
4. The excerpt is not expanded

Since the `buffer_ids_for_range` function basically did the same and had
even been changed the same way earlier I DRYed these functions as well.

Note: I'm a rust novice, so keep an extra eye on rust technicalities
when reviewing :)

---

Release Notes:

- Fix editor: expand excerpts failing when cursor is at excerpt start

---------

Co-authored-by: Lukas Wirth <me@lukaswirth.dev>

crates/multi_buffer/src/multi_buffer.rs

@@ -3616,27 +3616,10 @@ impl MultiBufferSnapshot {
         })
     }
 
-    pub fn excerpt_ids_for_range<T: ToOffset>(
+    fn excerpts_for_range<T: ToOffset>(
         &self,
         range: Range<T>,
-    ) -> impl Iterator<Item = ExcerptId> + '_ {
-        let range = range.start.to_offset(self)..range.end.to_offset(self);
-        let mut cursor = self.cursor::<MultiBufferOffset, BufferOffset>();
-        cursor.seek(&range.start);
-        std::iter::from_fn(move || {
-            let region = cursor.region()?;
-            if region.range.start >= range.end {
-                return None;
-            }
-            cursor.next_excerpt();
-            Some(region.excerpt.id)
-        })
-    }
-
-    pub fn buffer_ids_for_range<T: ToOffset>(
-        &self,
-        range: Range<T>,
-    ) -> impl Iterator<Item = BufferId> + '_ {
+    ) -> impl Iterator<Item = &Excerpt> + '_ {
         let range = range.start.to_offset(self)..range.end.to_offset(self);
         let mut cursor = self.cursor::<MultiBufferOffset, BufferOffset>();
         cursor.seek(&range.start);
@@ -3648,10 +3631,25 @@ impl MultiBufferSnapshot {
                 return None;
             }
             cursor.next_excerpt();
-            Some(region.excerpt.buffer_id)
+            Some(region.excerpt)
         })
     }
 
+    pub fn excerpt_ids_for_range<T: ToOffset>(
+        &self,
+        range: Range<T>,
+    ) -> impl Iterator<Item = ExcerptId> + '_ {
+        self.excerpts_for_range(range).map(|excerpt| excerpt.id)
+    }
+
+    pub fn buffer_ids_for_range<T: ToOffset>(
+        &self,
+        range: Range<T>,
+    ) -> impl Iterator<Item = BufferId> + '_ {
+        self.excerpts_for_range(range)
+            .map(|excerpt| excerpt.buffer_id)
+    }
+
     pub fn ranges_to_buffer_ranges<T: ToOffset>(
         &self,
         ranges: impl Iterator<Item = Range<T>>,