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/edit_prediction_context/src/tree_sitter_index.rs

@@ -506,7 +506,6 @@ mod tests {
     use super::*;
     use std::{path::Path, sync::Arc};
 
-    use futures::channel::oneshot;
     use gpui::TestAppContext;
     use indoc::indoc;
     use language::{Language, LanguageConfig, LanguageId, LanguageMatcher, tree_sitter_rust};
@@ -655,17 +654,10 @@ mod tests {
             expect_file_decl("a.rs", &decls[1], &project, cx);
         });
 
-        // Drop the buffer and wait for release
-        let (release_tx, release_rx) = oneshot::channel();
-        cx.update(|cx| {
-            cx.observe_release(&buffer, |_, _| {
-                release_tx.send(()).ok();
-            })
-            .detach();
+        // Need to trigger flush_effects so that the observe_release handler will run.
+        cx.update(|_cx| {
+            drop(buffer);
         });
-        drop(buffer);
-        cx.run_until_parked();
-        release_rx.await.ok();
         cx.run_until_parked();
 
         index.read_with(cx, |index, cx| {