[Client] Speak the 2026-07-28 lifecycle (#456)

* [Client] Speak the 2026-07-28 lifecycle

On that revision the client opens with server/discover instead of
initialize, stamps each request's _meta with the protocol version, its own
capabilities and client info, and sends the standard Mcp-Method / Mcp-Name /
Mcp-Param-* headers an intermediary routes on. ToolCatalog is what knows,
from the tool list, which arguments a call has to mirror.

An input_required result is answered rather than surfaced: InputRequestResolver
asks the host's elicitation, sampling and roots handlers and retries the same
request carrying inputResponses and the requestState the server sent.

McpHeader holds the header names and the =?base64?…?= sentinel, so the two
sides no longer keep their own copies of the same rules.

* Fix CI baseline drift and address Copilot review findings

Drop stale expected-failure entries the new client fixture now passes.

Copilot review: fix McpHeader round-trip for a wrapper-shaped literal,
encode empty/numeric-keyed inputResponses as a JSON object, stop the
incident example repeating side effects on MRTR retry, and reset
ToolCatalog on reconnect instead of carrying the previous server's
verdicts.

* Skip DualEraExampleTestCase on PHP 8.1

PHP_CLI_SERVER_WORKERS does not reliably fork multiple php -S
workers there (php/php-src#9400), reproducing the single-worker
deadlock the extra workers exist to avoid.

src/Schema/Request/DiscoverRequest.php

@@ -0,0 +1,43 @@
+<?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\Request;
+
+use Mcp\Schema\JsonRpc\Request;
+
+/**
+ * Asks a server what it is and what it speaks (SEP-2575).
+ *
+ * The modern era's optional replacement for `initialize`: it reports rather
+ * than negotiates, so a client MAY call it to learn the server's capabilities
+ * up front, and MAY skip it and simply declare a revision on each request.
+ *
+ * @see \Mcp\Schema\Result\DiscoverResult
+ *
+ * @author Christopher Hertel <mail@christopher-hertel.de>
+ */
+final class DiscoverRequest extends Request
+{
+    public static function getMethod(): string
+    {
+        return 'server/discover';
+    }
+
+    protected static function fromParams(?array $params): static
+    {
+        return new self();
+    }
+
+    protected function getParams(): ?array
+    {
+        return null;
+    }
+}