Add service extension for sending a sampling request to the MCP server

[elliette]

Work towards #326

[github-actions]

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[jakemac53]

Looks good but see my suggestion about simplifying this a bunch

[jakemac53]

Oh I just also realized this should probably be guarded behind a check that the client actually supports sampling, something like:

Suggested change

	await dtd.registerService(
	McpServiceConstants.serviceName,
	McpServiceConstants.samplingRequest,
	_handleSamplingRequest,
	);
	if (clientCapabilities.sampling != null) {
	await dtd.registerService(
	McpServiceConstants.serviceName,
	McpServiceConstants.samplingRequest,
	_handleSamplingRequest,
	);
	}

[jakemac53]

Also, do we need to make sure this service isn't already registered on DTD by some other client?

[elliette]

Oof good point! Done.

Re checking if another client has already registered it - can there be multiple Dart MCP servers connected to the same DTD connection? I didn't think there could. I don't know why another client would have registered it on DTD.

[jakemac53]

There can be yes - if multiple chat clients connect to the DTD instance started by an IDE.

For instance if somebody has two instances of Gemini CLI running at once for some reason, or two different chat bots.

[jakemac53]

Also just to clarify I am fine with landing this as is and following up with something to handle multiple clients trying to register the same method, if that is easier

[elliette]

Also just to clarify I am fine with landing this as is and following up with something to handle multiple clients trying to register the same method, if that is easier

Good because that was my plan 😄