[Server] Make subscriptions/listen actually deliver (#451)

The stream acknowledged, slept thirty seconds and closed, so tools and
resource changes never reached a client — and with resources/subscribe
gone, that left the revision with no server-push at all. A notification bus
carries them: in-memory where publisher and stream share a process, PSR-16
where php-fpm puts them in different ones. The conformance server proves
the latter — a tools/call in one worker reaches a stream held in another.

src/Server/Subscription/InMemoryNotificationBus.php

@@ -0,0 +1,74 @@
+<?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\Subscription;
+
+use Mcp\Exception\InvalidArgumentException;
+use Mcp\Schema\JsonRpc\Notification;
+
+/**
+ * A notification bus held in one process's memory.
+ *
+ * Correct for stdio and for persistent runtimes where the whole server lives in
+ * one process (Swoole, FrankenPHP worker mode, ReactPHP). Under PHP-FPM it is
+ * not: the worker that publishes and the worker holding the stream open are
+ * different processes, and a notification published in one is invisible to the
+ * other. {@see Psr16NotificationBus} is the answer there.
+ *
+ * @author Christopher Hertel <mail@christopher-hertel.de>
+ */
+final class InMemoryNotificationBus implements NotificationBusInterface
+{
+    /** @var array<int, Notification> */
+    private array $entries = [];
+
+    private int $next = 0;
+
+    /**
+     * @param int $backlog how many notifications to keep before dropping the oldest
+     */
+    public function __construct(
+        private readonly int $backlog = 256,
+    ) {
+        if ($this->backlog < 1) {
+            throw new InvalidArgumentException(\sprintf('The notification backlog must be at least one entry, got %d.', $this->backlog));
+        }
+    }
+
+    public function publish(Notification $notification): void
+    {
+        $this->entries[$this->next] = $notification;
+        ++$this->next;
+
+        // A stream that went away must not make this grow forever.
+        if (\count($this->entries) > $this->backlog) {
+            $this->entries = \array_slice($this->entries, -$this->backlog, preserve_keys: true);
+        }
+    }
+
+    public function cursor(): int
+    {
+        return $this->next;
+    }
+
+    public function since(int $cursor): array
+    {
+        $found = [];
+
+        foreach ($this->entries as $sequence => $notification) {
+            if ($sequence >= $cursor) {
+                $found[] = $notification;
+            }
+        }
+
+        return [$found, $this->next];
+    }
+}