chore: add changeset for connection close fix

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: EtanHey

.changeset/fix-connection-closed.md

@@ -0,0 +1,6 @@
+---
+'@modelcontextprotocol/core': patch
+'@modelcontextprotocol/server': patch
+---
+
+Fix unhandled promise rejections on transport close and detect stdin EOF in StdioServerTransport. Pending request promises are now rejected asynchronously via microtask deferral, and the server transport listens for stdin `end` events to trigger a clean shutdown when the client process exits.