Completion complete

[cafalchio]

Added support for completions.
Conformance test is passing.

Completion tests are still marked are ignored as they are dependent on list_prompt and list_resources.

[lucarlig]

Requesting changes: completion routing uses the wrong table, rewrites the wrong field, and is not advertised through discovery; please also enable a positive dataplane-only test.

[lucarlig]

Completion references are keyed in the prompt/resource routing tables, so looking them up in tools makes every valid completion return tool not found.

Suggested change

	let Some(downstream_name) = (match &request.r#ref {
	Reference::Prompt(_) => request.r#ref.as_prompt_name(),
	Reference::Resource(_) => request.r#ref.as_resource_uri(),
	_ => None,
	}) else {
	return Err(ErrorData {
	code: ErrorCode::INVALID_PARAMS,
	message: "Routing problem... completion not found".into(),
	data: None,
	});
	};
	let Some(route) = virtual_host.tools.get(downstream_name) else {
	return Err(ErrorData {
	code: ErrorCode::INVALID_PARAMS,
	message: "Routing problem... tool not found".into(),
	data: None,
	});
	};
	let route = match &request.r#ref {
	Reference::Prompt(prompt) => virtual_host.prompts.get(&prompt.name),
	Reference::Resource(resource) => virtual_host
	.resource_templates
	.get(&resource.uri)
	.or_else(|| virtual_host.resources.get(&resource.uri)),
	_ => None,
	};
	let Some(route) = route else {
	return Err(ErrorData {
	code: ErrorCode::INVALID_PARAMS,
	message: "Routing problem... completion not found".into(),
	data: None,
	});
	};

[lucarlig]

argument.name is the parameter being completed, so replacing it with the backend identifier leaves the namespaced ref unchanged and makes the backend reject the request.

Suggested change

	routed_request.argument.name = completion_name;
	match &mut routed_request.r#ref {
	Reference::Prompt(prompt) => prompt.name = completion_name,
	Reference::Resource(resource) => resource.uri = completion_name,
	_ => {},
	}

[lucarlig]

server/discover currently advertises no completions capability, so capability-aware clients will never invoke this handler.