Fix panic when a file in a path-based multibuffer excerpt is renamed (#28364)

Closes #ISSUE

Release Notes:

- Fixed a panic that could occur when paths changed in the project diff.

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

crates/text/src/text.rs

@@ -2231,6 +2231,7 @@ impl BufferSnapshot {
         } else if *anchor == Anchor::MAX {
             self.visible_text.len()
         } else {
+            debug_assert!(anchor.buffer_id == Some(self.remote_id));
             let anchor_key = InsertionFragmentKey {
                 timestamp: anchor.timestamp,
                 split_offset: anchor.offset,