docs(ai-chat): tag SSE reconnection entry with chat-prerelease-20260502065709

Replace the Upcoming / pending placeholder with the actual prerelease
version + date now that the publish has landed.

Author: ericallam

docs/ai-chat/changelog.mdx

@@ -4,7 +4,7 @@ sidebarTitle: "Changelog"
 description: "Pre-release updates for AI chat agents."
 ---
 
-<Update label="Upcoming" description="Pending next chat-prerelease publish" tags={["SDK"]}>
+<Update label="May 2, 2026" description="0.0.0-chat-prerelease-20260502065709" tags={["SDK"]}>
 
 ## Resilient SSE reconnection