[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/Schema/Result/DiscoverResult.php

@@ -0,0 +1,75 @@
+<?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\Schema\Result;
+
+use Mcp\Exception\InvalidArgumentException;
+use Mcp\Schema\Enum\ProtocolVersion;
+use Mcp\Schema\Enum\ResultType;
+use Mcp\Schema\JsonRpc\ResultInterface;
+use Mcp\Schema\ServerCapabilities;
+
+/**
+ * The server's response to a `server/discover` request.
+ *
+ * This is the modern era's replacement for `InitializeResult`. The difference
+ * is not just the method name: `initialize` *negotiates* a single version and
+ * remembers it for the connection, whereas `server/discover` merely *reports*
+ * every version this server speaks and lets each subsequent request pick one.
+ *
+ * Server identity travels in the result `_meta` rather than in a body
+ * `serverInfo` member (spec PR #3002 removed the latter).
+ *
+ * @author Christopher Hertel <mail@christopher-hertel.de>
+ */
+class DiscoverResult implements ResultInterface
+{
+    /**
+     * @param list<ProtocolVersion> $supportedVersions every revision this server can answer
+     */
+    public function __construct(
+        public readonly array $supportedVersions,
+        public readonly ServerCapabilities $capabilities,
+        public readonly ?string $instructions = null,
+    ) {
+        if ([] === $this->supportedVersions) {
+            throw new InvalidArgumentException('A DiscoverResult must advertise at least one supported version.');
+        }
+    }
+
+    /**
+     * Carries no `resultType`, caching hints or serverInfo identity: those are
+     * wire vocabulary, stamped by {@see \Mcp\Server\Wire\Rev2026Codec} on the
+     * way out. What is modelled here is only what discovery actually answers.
+     *
+     * @return array{
+     *     supportedVersions: list<string>,
+     *     capabilities: ServerCapabilities,
+     *     instructions?: string,
+     * }
+     */
+    public function jsonSerialize(): array
+    {
+        $data = [
+            'supportedVersions' => array_values(array_map(
+                static fn (ProtocolVersion $version): string => $version->value,
+                $this->supportedVersions,
+            )),
+            'capabilities' => $this->capabilities,
+        ];
+
+        if (null !== $this->instructions) {
+            $data['instructions'] = $this->instructions;
+        }
+
+        return $data;
+    }
+}