text: Assert text anchor offset validity on construction (#38441)

Attempt to aid debugging some utf8 indexing issues

Release Notes:

- N/A

Co-authored-by: Mikayla Maki <mikayla@zed.dev>

crates/multi_buffer/src/multi_buffer.rs

@@ -5259,7 +5259,8 @@ impl MultiBufferSnapshot {
             } else {
                 Anchor::max()
             };
-            // TODO this is a hack, remove it
+
+            // TODO this is a hack, because all APIs should be able to handle ExcerptId::min and max.
             if let Some((excerpt_id, _, _)) = self.as_singleton() {
                 anchor.excerpt_id = *excerpt_id;
             }