Make anchor_in_excerpt Optional (#8975)

We were seeing panics due to callers assuming they had valid
excerpt_ids, but that cannot easily be guaranteed across await points as
anyone may remove an excerpt.

Release Notes:

- Fixed a panic when hovering in a multibuffer

crates/assistant/src/assistant_panel.rs

@@ -2413,7 +2413,9 @@ impl ConversationEditor {
                 .read(cx)
                 .messages(cx)
                 .map(|message| BlockProperties {
-                    position: buffer.anchor_in_excerpt(excerpt_id, message.anchor),
+                    position: buffer
+                        .anchor_in_excerpt(excerpt_id, message.anchor)
+                        .unwrap(),
                     height: 2,
                     style: BlockStyle::Sticky,
                     render: Arc::new({