[Major-4] Multi Round-Trip Requests (MRTR) and required resultType #5682

Part of #5677

Spec link: https://modelcontextprotocol.io/specification/draft/changelog
Comprised of: Major change 7 (MRTR pattern) + Major change 8 (required resultType) — SEP-2322 — plus Minor change 11 (remove notifications/elicitation/complete and elicitationId, a direct consequence of MRTR)

What the spec says

• MRTR replaces server-initiated requests such as roots/list, sampling/createMessage, and elicitation/create.
• Servers return an InputRequiredResult (resultType: "input_required") whose inputRequests field carries the requests for additional information needed to process the request. Clients respond with inputResponses on a retry of the original request.
• All results now carry a required resultType field: "complete" for ordinary results, "input_required" for MRTR interim results. Clients MUST treat results from earlier-protocol servers that omit the field as "complete".
• notifications/elicitation/complete and the elicitationId field of URL-mode elicitation requests (both introduced in 2025-11-25) are removed: under MRTR the client learns the outcome of an out-of-band interaction by retrying the original request, so a server-initiated completion signal no longer fits. Servers needing to correlate an elicitation across retries encode their own identifier in requestState.

Gateway work

• Handle input_required results from modern upstreams; surface inputRequests to modern clients; forward retries carrying inputResponses to the correct upstream.
• Stamp resultType on all gateway-emitted results; normalize legacy upstream results (missing field) to "complete".
• Bridge design (the hard part): legacy upstream server-initiated requests (elicitation/create, sampling/createMessage, roots/list) ↔ modern client MRTR retry loop. Document per-feature feasibility and limitations; coordinate scope with [Deprecated-1] Roots, Sampling, and Logging deprecation #5690 (Sampling/Roots are deprecated, which may allow scoping bridging out).
• Remove notifications/elicitation/complete handling; pass requestState through opaquely and unmodified across retries.

Acceptance criteria

• MRTR round-trips work end-to-end modern↔modern through the gateway.
• All gateway results carry a valid resultType; legacy results normalized to "complete".
• requestState passes through untouched across retries.
• Bridging limitations for legacy server-initiated requests documented.