Pass Summary::Context to Item::summarize (#18510)

We are going to use this in the multi-buffer to produce a summary for an
`Excerpt` that contains a `Range<Anchor>`.

Release Notes:

- N/A

Co-authored-by: Nathan <nathan@zed.dev>

crates/sum_tree/src/tree_map.rs

@@ -224,7 +224,7 @@ where
 {
     type Summary = MapKey<K>;
 
-    fn summary(&self) -> Self::Summary {
+    fn summary(&self, _cx: &()) -> Self::Summary {
         self.key()
     }
 }