go-sdk/mcp: add mcp app example

[yurivish]

Adds a simple example of an MCP server that can display a user interface using the MCP app extension.

Fixes #815 (comment)

The tricky / somewhat inelegant bit is that the JS for the MCP Apps SDK is inlined into the HTML; it is possible to load other resources but they must be served from a web server, so I opted for simplicity to embed the required JS in index.html. Thinking about it more, this may not have been a great choice for an example. In my slightly more real-world example, I actually bundle and inline the code dynamically using esbuild's Go API.

[maciej-kisiel]

Thanks for the contribution @yurivish! I'm happy that you found some time to show how MCP Apps can be used to others.

I agree with the concern about the embedded JS code. I'm not very familiar with front-end development, but it would be great if there was a way to not put all the code in the example. Isn't the Apps SDK hosted somewhere so that it could be included as an external script?

I think the example could also use a small README.md to explain how to run it. I assume this is supposed to show a webpage, but it's not clear how open it given that the server uses STDIO transport.

There are also some simple fixes needed as uncovered by the test runs.