Capability discovery and list caching

[g2vinay]

Objective: Align discovery surfaces with server/discover, cache metadata, and the new client capability model.

Work items:

1. Determine how much of ServiceStartCommand and ServiceCollectionExtensions should move to capability discovery versus static server startup metadata.
2. Add or adapt the runtime to expose server discovery in a way compatible with the RC SDK.
3. Inspect list responses for opportunities to include ttlMs and cacheScope, especially for stable tool and namespace discovery surfaces.
4. Review tools/list and any derived discovery result caching in core/Microsoft.Mcp.Core/src/Areas/Server/Commands/ToolLoading for protocol-level cache metadata exposure.
5. Keep server-side caches if they are still valuable, but do not rely on them to represent protocol freshness semantics.

Acceptance criteria:

• Discovery works without the initialize handshake.
• Tool list surfaces advertise freshness accurately when the SDK supports it.
• Caching behavior is explicit and testable.