[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.

tests/Conformance/Fixtures/nginx.conf

@@ -3,12 +3,10 @@ server {
     server_name localhost;
     root /app;
 
-    # / speaks the handshake era, /stateless the modern one (SEP-2575).
-    # Declared first so the prefix match wins.
-    location /stateless {
-        try_files $uri /tests/Conformance/server-stateless.php$is_args$args;
-    }
-
+    # One entry, both protocol eras: the fixture is built through
+    # Builder::build(), which carries a dispatcher for each, and the transport
+    # routes every request to the one it belongs to. Both conformance suites
+    # run against this same location.
     location / {
         try_files $uri /tests/Conformance/server.php$is_args$args;
     }