docs: [LangGraph Server Changelog Bot] Changelog updates for new version(s) (#506)

Automated changelog update created by the LangGraph Server Changelog
Bot.

Feel free to merge anytime.

Author: samecrowder

src/langgraph-platform/langgraph-server-changelog.mdx

@@ -5,6 +5,10 @@ sidebarTitle: Server changelog
 
 [LangGraph Server](/langgraph-platform/langgraph-server) is an API platform for creating and managing agent-based applications. It provides built-in persistence, a task queue, and supports deploying, configuring, and running assistants (agentic workflows) at scale. This changelog documents all notable updates, features, and fixes to LangGraph Server releases.
 
+<a id="2025-09-11"></a>
+## v0.4.20
+- Optimized Pubsub initialization to prevent overhead and address subscription timing issues, ensuring smoother run execution.
+
 <a id="2025-09-11"></a>
 ## v0.4.19
 - Removed warnings from psycopg by addressing function checks introduced in version 3.2.10.