mcp: support per-resource cache policy for resources/read

[makeavish]

Is your feature request related to a problem? Please describe.

A server may expose both public static resources and tenant-specific resource templates. Today, assigning the correct ttlMs and cacheScope to each resources/read result requires receiving middleware—or the proposed DefaultCacheable callback in #1128—to inspect the request URI. That duplicates policy already known where the resource is registered.

Describe the solution you'd like

Allow direct resources and resource templates to declare a cache hint at registration. The matched resource's hint should apply only to its resources/read result, take precedence over the server default, and not appear in resources/list or resources/templates/list.

For example, a server could register static documentation as public and a tenant-backed template as private without central URI matching.

The TypeScript SDK already supports this through cacheHint on registerResource.

Describe alternatives you've considered

• Receiving middleware works, but moves resource-specific policy away from registration.
• mcp: add ServerOptions.DefaultCacheable #1128's server-level callback is useful for broad or request-dependent defaults, but registration-level policy would complement it for stable per-resource semantics.

Additional context

Related: #1094 and #1128.