Fix minor clean up of sigint

[cdcabrera]

As a maintainer I need to review and potentially perform some minor clean up how we handle server shutdown of the server instance. Technically it's a non-issue, but the potential for multiple handlers is there.

In server.ts

let sigintHandler: (() => void) | null = null;
...
const stopServer = async () => {
...
      if (sigintHandler) {
        process.off('SIGINT', sigintHandler);
        sigintHandler = null;
      }
...

    if (enableSigint && !sigintHandler) {
      sigintHandler = () => {
        void stopServer();
      };
     process.on('SIGINT', sigintHandler);

Acceptance criteria

• Reviewed the potential for multiple handlers, if necessary apply patch
• Popped up in work for MCP tools should be expandable via tool-plugins #22