Dual-era server: requests that carry io.modelcontextprotocol/protocolVersion are served statelessly (server/discover
with supportedVersions, capabilities, instructions, ttlMs / cacheScope; resultType complete and serverInfo in every
result's _meta; cacheable tools/list; -32022 UnsupportedProtocolVersion with data { supported, requested }; -32602 when
clientCapabilities is missing). initialize / ping / unannotated requests keep the 2025-11-25 (and older) behaviour.
Tests for both handshakes, including one stdio process serving a modern probe and a legacy client.
