Fix panic folding in multi-buffers (#21511)

Closes #19054

Rename `max_buffer_row()` to `widest_line_number()` to (hopefully)
prevent
people assuming it means the same as `max_point().row`.

Release Notes:

- Fixed a panic when folding in a multibuffer

crates/editor/src/display_map/inlay_map.rs

@@ -1019,7 +1019,7 @@ impl InlaySnapshot {
         let inlay_point = InlayPoint::new(row, 0);
         cursor.seek(&inlay_point, Bias::Left, &());
 
-        let max_buffer_row = MultiBufferRow(self.buffer.max_point().row);
+        let max_buffer_row = self.buffer.max_row();
         let mut buffer_point = cursor.start().1;
         let buffer_row = if row == 0 {
             MultiBufferRow(0)