Fix stale buffer snapshot used in diagnostics multibuffer (#52937)

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [ ] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- N/A

Co-authored-by: Conrad <conrad@zed.dev>

crates/diagnostics/src/diagnostics.rs

@@ -624,6 +624,7 @@ impl ProjectDiagnosticsEditor {
                         });
                     })
                 }
+                let buffer_snapshot = buffer.read(cx).snapshot();
                 let excerpt_ranges: Vec<_> = excerpt_ranges
                     .into_iter()
                     .map(|range| ExcerptRange {