fix(app-server): unload threads from shared processor

Allow thread teardown to run from shared connection handlers by making unload_thread_without_subscribers take &self. The method only uses interior-mutability fields and cloned handles, so it does not need mutable access to the processor.

Co-authored-by: Codex noreply@openai.com

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

@@ -5601,7 +5601,7 @@ impl CodexMessageProcessor {
     }
 
     async fn unload_thread_without_subscribers(
-        &mut self,
+        &self,
         thread_id: ThreadId,
         thread: Arc<CodexThread>,
     ) {