fix: memory leak in SimpleCache causing MCP connection loss (fixes #118)

- Added cleanupTimer property to track setInterval timer
- Implemented destroy() method to clear timer and prevent memory leak
- Updated MCP server shutdown to call cache.destroy()
- Enhanced HTTP server error handling with transport.onerror
- Fixed event listener cleanup to prevent accumulation
- Added comprehensive test coverage for memory leak prevention

This fixes the issue where MCP server would lose connection after
several hours due to timer accumulation causing memory exhaustion.

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

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

src/mcp/server.ts

@@ -2538,6 +2538,16 @@ Full documentation is being prepared. For now, use get_node_essentials for confi
   async shutdown(): Promise<void> {
     logger.info('Shutting down MCP server...');
     
+    // Clean up cache timers to prevent memory leaks
+    if (this.cache) {
+      try {
+        this.cache.destroy();
+        logger.info('Cache timers cleaned up');
+      } catch (error) {
+        logger.error('Error cleaning up cache:', error);
+      }
+    }
+    
     // Close database connection if it exists
     if (this.db) {
       try {