fix: memory leak in session removal - close MCP server properly (#471) (#472)

- Add close() method to N8NDocumentationMCPServer that:
  - Calls server.close() (MCP SDK cleanup)
  - Clears internal cache
  - Nullifies service references to help GC
- Update removeSession() to call server.close() before releasing references

Root cause: removeSession() deleted server from map but didn't call cleanup
Evidence: Production server memory grew ~1GB in 43 minutes (10% to 35%)

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

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Romuald Członkowski <romualdczlonkowski@MacBook-Pro-Romuald.local>
Co-authored-by: Claude <noreply@anthropic.com>

package.runtime.json

@@ -1,6 +1,6 @@
 {
   "name": "n8n-mcp-runtime",
-  "version": "2.28.0",
+  "version": "2.28.7",
   "description": "n8n MCP Server Runtime Dependencies Only",
   "private": true,
   "dependencies": {