implementing SEP-2549

[michaelneale]

implementing #875

(assisted by mic and goose)

[alexhancock]

Pre-approving so you can merge when the below is addressed!

The cache hints (ttlMs and cacheScope) are written to _meta, but the spec defines them as first-class result fields

Per SEP-2549, ttlMs and cacheScope are properties on the result itself (CacheableResult), so the wire shape is { "tools": [...], "ttlMs": 300000, "cacheScope": "public" } (example, L112) — not nested under _meta

Fix list:

• Top-level fields: make ttl_ms/cache_scope real #[serde(rename = "ttlMs"/"cacheScope")] fields on the result struct instead of meta.insert(...).
• resources/read: the same meta.insert pattern is duplicated below in ReadResourceResult, writing into each ResourceContents. The spec attaches the hints to ReadResourceResult itself, so they should live on the result (otherwise they vanish when contents is empty). Scope guidance L90.
• ttlMs rules: u64 correctly blocks negatives on the producing side 👍, but there's no read-side normalization (absent → 0, negative → 0, server MUST emit >= 0). Rules L39-43.

[alexhancock]

https://modelcontextprotocol.io/seps/2549-TTL-for-list-results#cache-scope

This should have "public" and "private"

[jamadeo]

I think this does the right thing over the wire, but wouldn't it be a good bit simpler if added on the top level struct itself?

[jamadeo]

Why not add as new fields here and avoid the custom implementations of serialization/schema?