[Major-1] Dual-stack stateless core protocol #5679

Part of #5677

Spec link: https://modelcontextprotocol.io/specification/2026-07-28/changelog
Comprised of: Major change 2 (remove initialize handshake) + Major change 3 (server/discover) — SEP-2575

What the spec says

• The initialize/notifications/initialized handshake is removed entirely.
• Every request now carries its protocol version and client capabilities in _meta under the keys io.modelcontextprotocol/protocolVersion and io.modelcontextprotocol/clientCapabilities.
• Clients SHOULD identify themselves on each request via io.modelcontextprotocol/clientInfo; servers SHOULD identify themselves in each result's _meta via io.modelcontextprotocol/serverInfo.
• Version mismatches return UnsupportedProtocolVersionError (renumbered to -32022 — see [Minor-3] Error code alignment #5686).
• Servers MUST implement the new server/discover RPC to advertise their supported protocol versions, capabilities, and identity. Clients MAY call it before any other request for up-front version selection, or use it as a backward-compatibility probe on STDIO.

Gateway work (dual-stack)

• Dialect detection on the client-facing side: first request initialize → legacy path (respond with InitializeResult, mint facade session); request carrying _meta protocol version or server/discover → modern stateless path.
• Upstream probing: call server/discover on connect; success → modern upstream (cache versions/capabilities in registry with a dialect field and re-probe on errors); method-not-found → legacy upstream, run the handshake with a gateway-owned session.
• Translation core: modern client → legacy upstream (strip _meta protocol fields, map per-request capabilities onto the connect-time session); legacy client → modern upstream (fabricate InitializeResult from cached server/discover data, inject the three _meta client keys on every forwarded request, return serverInfo mapping).
• Capability reconciliation layer between server/discover and InitializeResult shapes; filter capabilities that cannot be bridged across dialects.
• Implement server/discover (MUST) on the gateway's server side, advertising both dialects.

Acceptance criteria

• All four dialect combinations (legacy/modern client × legacy/modern upstream) pass integration tests.
• server/discover returns correct versions, capabilities, and identity.
• _meta validated on every modern-path request; -32022 returned on unsupported versions.
• No behavioral change for existing legacy clients.