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>

dist/mcp/server.d.ts

@@ -14,6 +14,7 @@ export declare class N8NDocumentationMCPServer {
     private earlyLogger;
     private disabledToolsCache;
     constructor(instanceContext?: InstanceContext, earlyLogger?: EarlyErrorLogger);
+    close(): Promise<void>;
     private initializeDatabase;
     private initializeInMemorySchema;
     private parseSQLStatements;