[Server][Client] SEP-2575: Stateless protocol (per-request version, server/discover, messages/listen)

[chr-hertel]

Implements SEP-2575 for the MCP Spec 2026-07-28 release.

Tracked by umbrella #334.

Spec summary

Removes the mandatory initialize/initialized handshake. Protocol version and client info travel in _meta on every request; servers can use server/discover to fetch capabilities. Adds messages/listen RPC replacing the GET-SSE channel.

PHP SDK changes

• src/Server/Handler/Request/InitializeHandler.php, src/Schema/Request/InitializeRequest.php, src/Schema/Result/InitializeResult.php — keep working for old clients but make optional; gate on protocol version.
• src/Server/Protocol.php — drop hard requirement that initialize precedes other calls; route server/discover and messages/listen.
• Add src/Server/Handler/Request/DiscoverHandler.php and MessagesListenHandler.php; corresponding schemas under src/Schema/Request|Result/.
• src/Server/Transport/Http/Middleware/ProtocolVersionMiddleware.php — extend to also read _meta.protocolVersion from JSON-RPC payloads and emit UnsupportedVersionError.
• src/Server/Transport/StreamableHttpTransport.php — route streaming through messages/listen POST; legacy GET-SSE deprecated.
• src/Client/Protocol.php and src/Client/Builder.php — skip handshake when negotiating new version; emit per-request capabilities/version; handle UnsupportedVersionError retry.

Breaking change. Coordinate with deprecation policy (SEP-2596, umbrella #339).

Related SEPs

• SEP-2567 (umbrella [2026-07-28] Stateless Core overhaul #334) — companion sessionless work
• SEP-2322 (umbrella [2026-07-28] Stateless Core overhaul #334) — Multi Round-Trip Requests