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/channel/src/channel_chat.rs

@@ -808,7 +808,7 @@ pub fn mentions_to_proto(mentions: &[(Range<usize>, UserId)]) -> Vec<proto::Chat
 impl sum_tree::Item for ChannelMessage {
     type Summary = ChannelMessageSummary;
 
-    fn summary(&self) -> Self::Summary {
+    fn summary(&self, _cx: &()) -> Self::Summary {
         ChannelMessageSummary {
             max_id: self.id,
             count: 1,