[Server][Tests] Serve the modern (SEP-2575) lifecycle (#444)

* [Server][Tests] Serve the modern (SEP-2575) lifecycle

The revision removed the initialize handshake: every request carries its own
_meta - protocol version, client capabilities, client info - and answers
server/discover for what initialize used to return. There is no session to
keep, so there is nothing for the handshake-era dispatcher to do.

Builder::buildStateless() produces StatelessProtocol, and
StatelessHttpTransport serves it over one POST-only endpoint - GET and
DELETE are 405, and no Mcp-Session-Id exists to hand out.

Results go through a per-era wire codec rather than the handlers, so
resultType and the caching hints are stamped for 2026-07-28 and left off the
revisions that do not define them.

Covered by the modern conformance suite (make conformance-draft-server)
against its own baseline; unit coverage arrives with the fixes stacked on
this.

* Apply suggestions from code review

Co-authored-by: Christopher Hertel <mail@christopher-hertel.de>

* Fix critical stateless-lifecycle bugs from review

- Don't leak internal exception messages to clients on uncaught
  \Throwable; use a generic message like the handshake-era protocol
  and every built-in handler already do.
- Default a missing/invalid request id to null instead of "", and
  stop answering notifications and malformed messages as if they
  were "method not found" 404s.
- Read the POST body in a loop instead of a single read() call,
  which PSR-7 allows to return short.

src/Server/Wire/Rev2025Codec.php

@@ -0,0 +1,32 @@
+<?php
+
+/*
+ * This file is part of the official PHP MCP SDK.
+ *
+ * A collaboration between Symfony and the PHP Foundation.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Mcp\Server\Wire;
+
+/**
+ * The wire codec for every revision through 2025-11-25.
+ *
+ * The identity transform, and deliberately so. These revisions have no
+ * `resultType`, no caching hints and no `_meta` serverInfo requirement, so the
+ * neutral result body *is* the wire body — there is nothing to stamp and,
+ * more importantly, nothing newer that may leak in. A server negotiating one
+ * of these revisions emits exactly the bytes it emitted before the modern era
+ * existed.
+ *
+ * @author Christopher Hertel <mail@christopher-hertel.de>
+ */
+final class Rev2025Codec implements WireCodecInterface
+{
+    public function encodeResult(string $method, array $result): array
+    {
+        return $result;
+    }
+}