MCP 2026-07-28 goes stateless: initialize handshake removed

The MCP spec version 2026-07-28 makes the protocol stateless: the initialize/notifications/initialized handshake is gone, and every request now carries its protocol version and client capabilities in _meta. Servers must implement server/discover, and all results gain a resultType field plus ttlMs and cacheScope caching hints.

• Requests missing required _meta keys are rejected with error -32602
• Unsupported protocol versions return error -32022
• server/discover is now mandatory for all servers
• tools/list and server/discover results require ttlMs and cacheScope