SEP-2549: clarify whether cacheScope is required or defaults to public #2939

What's broken?

The spec is ambiguous or self-contradictory

Where in the spec or docs?

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

What should happen?

The spec should clearly define whether cacheScope is required on cacheable results, and what clients should do if it is absent.

In particular:

1. Should conforming servers always emit cacheScope?
2. If cacheScope is missing from an older or nonconforming server, should clients treat it as "public", "private", or not cacheable?
3. Should SDKs provide a default, or require server authors to choose explicitly?

What actually happens?

The SEP appears to say two different things.

https://modelcontextprotocol.io/seps/2549-TTL-for-list-results#schema-change-typescript

https://modelcontextprotocol.io/seps/2549-TTL-for-list-results#backward-compatibility

Anything else?

No response