fix: return 202 for notifications with stale sessions, warn on updateTable columns (#654)

**Issue #654 (critical):** handleRequest() returned HTTP 400 for JSON-RPC
notifications with stale/expired session IDs. Per JSON-RPC 2.0 spec,
notifications don't expect responses. Returning 400 caused Claude's proxy
to interpret the connection as broken, triggering reconnection storms
(930 errors/day, 216 users affected).

Fix: Added isJsonRpcNotification() helper that detects single and batch
notifications (messages with method but no id field). Path C in
handleRequest() now returns 202 Accepted for notifications with stale
sessions instead of 400. Also added TOCTOU null guard in Path B.

**Issue 1 (medium):** updateTable silently accepted columns parameter
via the shared MCP tool schema. Zod stripped it but gave no feedback.

Fix: handleUpdateTable now detects columns in raw args and appends a
warning to the response. Tool schema descriptions clarified that columns
are for createTable only and schema is immutable after creation.

Conceived by Romuald Członkowski - https://www.aiadvisors.pl/en

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

package.json

@@ -1,6 +1,6 @@
 {
   "name": "n8n-mcp",
-  "version": "2.40.2",
+  "version": "2.40.3",
   "description": "Integration between n8n workflow automation and Model Context Protocol (MCP)",
   "main": "dist/index.js",
   "types": "dist/index.d.ts",