Accept any JSON value as structuredContent in vMCP

[JAORMX]

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

Work

vMCP rejects tool results whose structuredContent is not a JSON object — arrays/primitives are "not supported" (pkg/vmcp/client/client.go:926). The 2026-07-28 revision (SEP-2106) explicitly allows structuredContent to be any JSON value, with schemas as full JSON Schema 2020-12.

• Accept any JSON value for structuredContent in vMCP's client/conversion path and forward it faithfully.
• Check the composer's output validation/construction for the same object-only assumption.

Small, self-contained. Can land before the rest of Workstream C.

[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.