acp: Fix agent servers sometimes not being registered when Zed starts (#38330)

In local projects, initialize the list of agents in the agent server
store immediately. Previously we were initializing the list only after a
delay, in an attempt to avoid sending the `ExternalAgentsUpdated`
message to the downstream client (if any) before its handlers were
initialized. But we already have a separate codepath for that situation,
in the `AgentServerStore::shared`, and we can insert the delay in that
place instead.

Release Notes:

- acp: Fixed a bug where starting an external agent thread soon after
Zed starts up would show a "not registered" error.

---------

Co-authored-by: Michael <michael@zed.dev>
Co-authored-by: Agus <agus@zed.dev>

crates/remote_server/src/headless_project.rs

@@ -197,7 +197,7 @@ impl HeadlessProject {
         let agent_server_store = cx.new(|cx| {
             let mut agent_server_store =
                 AgentServerStore::local(node_runtime.clone(), fs.clone(), environment, cx);
-            agent_server_store.shared(REMOTE_SERVER_PROJECT_ID, session.clone());
+            agent_server_store.shared(REMOTE_SERVER_PROJECT_ID, session.clone(), cx);
             agent_server_store
         });