[v2] Server: print warning if `sendResourceUpdated` is used but `subscribe` is not set

What happened?

I lost a few hours to realize that if I want to subscribe to a resource, I need to explicitly put in the server:

  const server = new McpServer({ name: 'proofdiag', version: '0.0.1' }, {
    capabilities: {
      tools: {},
      resources: { subscribe: true, listChanged: true }, // <--- this is the important line
    }
  });
  // Later someone calls
  await server.server.sendResourceUpdated({ uri }).catch((e) => {console.log("Error in sendResourceUpdated", e)});

Otherwise, when subscribing the client would just receive an acknowledgment with an empty notifications, which is really hard to debug. As such, I would expect maybe some warning/errors if the declared capabilities do not allow to call a functionality (here when calling sendResourceUpdated or when receiving from the client the subscribe message), or otherwise capabilities should be changed based on what the server defines.

What did you expect?

An error/warning.

Code to reproduce

MWE with resource subscription.

SDK version

2.0.0-beta.4

Area

Server

Start by tracing the server-side sendResourceUpdated call and the incoming resource subscribe handling described in the reproduction. Check how declared resource capabilities are represented and how failures are currently reported; done means an absent subscribe capability produces a clear warning or error instead of an empty acknowledgment.