Lay the groundwork to create terminals in AcpThread (#35872)

This just prepares the types so that it will be easy later to update a
tool call with a terminal entity. We paused because we realized we want
to simplify how terminals are created in zed, and so that warrants a
dedicated pull request that can be reviewed in isolation.

Release Notes:

- N/A

---------

Co-authored-by: Ben Brandt <benjamin.j.brandt@gmail.com>

crates/agent2/src/tools/find_path_tool.rs

@@ -94,8 +94,12 @@ impl AgentTool for FindPathTool {
         acp::ToolKind::Search
     }
 
-    fn initial_title(&self, input: Self::Input) -> SharedString {
-        format!("Find paths matching “`{}`”", input.glob).into()
+    fn initial_title(&self, input: Result<Self::Input, serde_json::Value>) -> SharedString {
+        let mut title = "Find paths".to_string();
+        if let Ok(input) = input {
+            title.push_str(&format!(" matching “`{}`”", input.glob));
+        }
+        title.into()
     }
 
     fn run(
@@ -111,7 +115,7 @@ impl AgentTool for FindPathTool {
             let paginated_matches: &[PathBuf] = &matches[cmp::min(input.offset, matches.len())
                 ..cmp::min(input.offset + RESULTS_PER_PAGE, matches.len())];
 
-            event_stream.send_update(acp::ToolCallUpdateFields {
+            event_stream.update_fields(acp::ToolCallUpdateFields {
                 title: Some(if paginated_matches.len() == 0 {
                     "No matches".into()
                 } else if paginated_matches.len() == 1 {