Support MCP caching metadata (ttlMs/cacheScope) in vMCP

[JAORMX]

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

Work

2026-07-28 requires caching metadata on list/read results: ttlMs + cacheScope (public/private) via CacheableResult (SEP-2549), and tools/list SHOULD be deterministically ordered for cache hits.

• vMCP: emit ttlMs/cacheScope on aggregated list results — natural fit, the caching aggregator already has a TTL; ensure deterministic tool ordering.
• vMCP client: respect backend-provided TTLs in the aggregation cache instead of a fixed TTL.
• Proxies: pass through untouched (verify nothing strips _meta/result fields).

Depends on: the go-sdk v1.7 adoption issue for types. Priority: opportunistic — pairs well with the vMCP stateless work.

[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.