Handling multiple elicitation requests #50

Related to #47 and the new elicitation API from #21. If concurrent tool execution can happen then we might have multiple elicitation requests via requestUserInteraction. How should those be handled? What's the expected behaviour if the callback passed to requestUserInteraction is running when another tool execution makes this request too?

The reason for having the elicitation API is two-fold:

• Asking the browser to bring user attention to this web content, in case it's not foreground.
• Asking the browser to re-enable user input to the web content, in case it's paused during the tool execution.

Given that, once we get into a "user input active" state the browser can keep dispatching these elicitation callbacks in parallel.