Rewrite server-initiated MCP calls as Multi Round-Trip Requests — return `input_required` and wait for the client to retry

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SEP-2322 replaces server-initiated requests (`roots/list`, `sampling/createMessage`, `elicitation/create`) with the MRTR pattern: the server returns an `InputRequiredResult` carrying `inputRequests`, and the client re-issues the original request with `inputResponses` attached. Every result now carries a required `resultType` field (`"complete"` or `"input_required"`), and clients MUST treat a missing field from older servers as complete. The practical consequence is that `notifications/elicitation/complete` and `elicitationId` are gone — servers needing to correlate an elicitation across retries must encode their own identifier in `requestState`.