Build a reference to a prompt argument.
Build a reference to a resource template URI.
Prompt name (for ref/prompt).
Optional human-readable display title for the prompt (ref/prompt only). PromptReference extends BaseMetadata, so a prompt reference may carry BaseMetadata's optional title. Has no meaning for ref/resource (ResourceTemplateReference does not extend BaseMetadata) and is never serialized in that case.
Either "ref/prompt" or "ref/resource".
Resource (template) URI (for ref/resource).
A completion/complete reference: the thing being completed. Per server/utilities/completion §"Requesting Completions", a client specifies either a prompt (ref/prompt, identified by name) or a resource template (ref/resource, identified by uri). Use forPrompt / forResource to construct one.