Usage of deprecated `A2AExpressApp` in the JS examples

[skurzyp]

What happened?

The samples/js samples uses the A2AExpressApp which is now deprecated

  // 4. Create and setup A2AExpressApp
  const appBuilder = new A2AExpressApp(requestHandler);
  const expressApp = appBuilder.setupRoutes(express());

The hint is to use:

Deprecated:
Use specific middlewares (jsonRpcHandler, agentCardHandler) directly.

TODO:
The examples should be updated to align with the newest package version v0.3.7.

Relevant log output

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[skurzyp]

Apparently similar issue is present also in Python examples and probably in other languages too. See #385