Design MRTR-based elicitation and sampling passthrough for vMCP

[JAORMX]

Part of #5743 (Workstream C: 2026-07-28 stateless readiness).

Work

vMCP has no sampling passthrough and no backend→client elicitation passthrough today (only composer-driven form elicitation; TODO at pkg/vmcp/client/client.go:833). Rather than building these on the 2025-11-25 server-initiated request model — deprecated in 2026-07-28 — design them MRTR-first:

• Backend returns resultType: "input_required" with inputRequests → vMCP surfaces it to the client → client retries with inputResponses → vMCP replays to the backend with correlation via requestState.
• Covers elicitation (form + URL modes) and sampling uniformly, since MRTR replaces both.
• For 2025-11-25 backends that still issue true server-initiated requests, either bridge them into MRTR toward 2026-07-28 clients or scope support to matching protocol pairs — decide in design.

Depends on: the vMCP stateless design issue. Supersedes building legacy sampling/elicitation passthrough.

[JAORMX]

Deferred: this work is downstream of the 2026-07-28 stateless design issues (#5755/#5756) and the spec/go-sdk going final. Listed in epic #5743 under expected follow-ups; will be re-filed when the design makes it actionable.