docs(ai-chat): mark SSE reconnection entry as Upcoming

Replace the <NEXT-PRERELEASE> placeholder with explicit Upcoming /
"Pending next chat-prerelease publish" framing so the docs can ship
ahead of the prerelease cut. Update at publish time with the actual
version label.

Author: ericallam

docs/ai-chat/changelog.mdx

@@ -4,7 +4,7 @@ sidebarTitle: "Changelog"
 description: "Pre-release updates for AI chat agents."
 ---
 
-<Update label="May 1, 2026" description="<NEXT-PRERELEASE>" tags={["SDK"]}>
+<Update label="Upcoming" description="Pending next chat-prerelease publish" tags={["SDK"]}>
 
 ## Resilient SSE reconnection