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/thinking_tool.rs

@@ -30,7 +30,7 @@ impl AgentTool for ThinkingTool {
         acp::ToolKind::Think
     }
 
-    fn initial_title(&self, _input: Self::Input) -> SharedString {
+    fn initial_title(&self, _input: Result<Self::Input, serde_json::Value>) -> SharedString {
         "Thinking".into()
     }
 
@@ -40,7 +40,7 @@ impl AgentTool for ThinkingTool {
         event_stream: ToolCallEventStream,
         _cx: &mut App,
     ) -> Task<Result<String>> {
-        event_stream.send_update(acp::ToolCallUpdateFields {
+        event_stream.update_fields(acp::ToolCallUpdateFields {
             content: Some(vec![input.content.into()]),
             ..Default::default()
         });