[Server] Serve both protocol eras from one endpoint (#455)

* [Server] Serve both protocol eras from one endpoint

The two lifecycles share a transport, not a dispatcher. StreamableHttpTransport
classifies each request - a 2026-07-28 envelope, an initialize handshake, or a
session-bound follow-up - through InboundClassifier and routes it to the
dispatcher that owns it, so one URL answers a modern client and a handshake-era
one alike. Builder::build() carries both; withoutModernEra() opts out and
setModernVersions() narrows what the modern leg answers for.

InputRequiredShim lets a handler written for multi round-trip requests also
serve a handshake-era client, by turning each ask into the request/response
exchange that era has - so a handler is written once rather than twice.

The conformance fixture collapses into one server for the same reason: both
legs now hit the same URL.

* Trim redundant comments, fix stale in-memory claim in conformance fixture

* Drop stale json-schema-2020-12 baseline entries

The new fixture tool makes this scenario pass in both eras now.

* Reject handshake revisions in setModernVersions(), warn on unsafe custom middleware

Copilot review: setModernVersions() accepted revisions InboundClassifier
would never route to the modern leg. StreamableHttpTransport now warns
when a custom middleware list carries ProtocolVersionMiddleware, since
it runs before era classification and rejects modern-era traffic by
default. Also fixed a misleading docblock claim on the same middleware.

src/Server/ClientGateway.php

@@ -403,14 +403,20 @@ class ClientGateway
      * This suspends the Fiber and waits for the client to respond. The transport
      * handles polling the session for the response and resuming the Fiber when ready.
      *
+     * Public for {@see InputRequiredShim}, which sends the requests a handler
+     * embedded in an {@see \Mcp\Schema\Result\InputRequiredResult} and knows
+     * nothing about their kinds. Prefer the typed methods above.
+     *
      * @param Request $request The request to send
      * @param int     $timeout Maximum time to wait for response (seconds)
      *
      * @return Response<array<string, mixed>>|Error The client's response message
      *
      * @throws RuntimeException If Fiber support is not available
+     *
+     * @internal
      */
-    private function request(Request $request, int $timeout = 120): Response|Error
+    public function request(Request $request, int $timeout = 120): Response|Error
     {
         $response = \Fiber::suspend([
             'type' => 'request',