fix: upgrade MCP SDK and fix messaging

- Upgrade @modelcontextprotocol/sdk from 0.6.1 to 1.25.3
- Fix handlePostMessage signature (now requires req.body)
- Fix agent registration to persist by name instead of sessionId
- Add session-to-agent mapping for notifications
- Enable logging capability for server notifications

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

package.json

@@ -25,7 +25,7 @@
   "author": "",
   "license": "MIT",
   "dependencies": {
-    "@modelcontextprotocol/sdk": "^0.6.0",
+    "@modelcontextprotocol/sdk": "^1.25.3",
     "@types/ws": "^8.18.1",
     "cors": "^2.8.5",
     "dotenv": "^16.4.5",