app-server: Silence thread status changes caused by thread being created (#13079)

Currently we emit `thread/status/changed` with `Idle` status right
before sending `thread/started` event (which also has `Idle` status in
it).
It feels that there is no point in that as client has no way to know
prior state of the thread as it didn't exist yet, so silence these kinds
of notifications.

codex-rs/app-server/src/codex_message_processor.rs

@@ -2198,7 +2198,7 @@ impl CodexMessageProcessor {
 
                 listener_task_context
                     .thread_watch_manager
-                    .upsert_thread(thread.clone())
+                    .upsert_thread_silently(thread.clone())
                     .await;
 
                 thread.status = resolve_thread_status(
@@ -3704,7 +3704,7 @@ impl CodexMessageProcessor {
         }
 
         self.thread_watch_manager
-            .upsert_thread(thread.clone())
+            .upsert_thread_silently(thread.clone())
             .await;
 
         thread.status = resolve_thread_status(
@@ -6295,7 +6295,7 @@ impl CodexMessageProcessor {
                 Ok(summary) => {
                     let mut thread = summary_to_thread(summary);
                     self.thread_watch_manager
-                        .upsert_thread(thread.clone())
+                        .upsert_thread_silently(thread.clone())
                         .await;
                     thread.status = resolve_thread_status(
                         self.thread_watch_manager