[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/Client/Stateless/HeaderFactory.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\Client\Stateless;
+
+use Mcp\Schema\Enum\ProtocolVersion;
+use Mcp\Schema\Wire\McpHeader;
+
+/**
+ * The HTTP headers a modern-era client puts on every POST (SEP-2243, SEP-2575).
+ *
+ * These exist so an intermediary can route and authorize MCP traffic without
+ * parsing the body. That only works if the headers cannot disagree with what
+ * they mirror, so they are derived here from the very message that is about to
+ * be sent — never from what the caller believes it is sending.
+ *
+ * @see \Mcp\Server\Stateless\StandardHeaderValidator the server that checks this
+ *
+ * @author Christopher Hertel <mail@christopher-hertel.de>
+ */
+final class HeaderFactory
+{
+    public function __construct(
+        private readonly ToolCatalog $tools,
+    ) {
+    }
+
+    /**
+     * @param array<string, mixed> $payload a serialized JSON-RPC message
+     *
+     * @return array<string, string>
+     */
+    public function forMessage(array $payload, ProtocolVersion $protocolVersion): array
+    {
+        $method = $payload['method'] ?? null;
+
+        // A response to a server-initiated request carries no method to mirror;
+        // the version header is unconditional and still applies.
+        if (!\is_string($method)) {
+            return [McpHeader::PROTOCOL_VERSION => $protocolVersion->value];
+        }
+
+        $params = \is_array($payload['params'] ?? null) ? $payload['params'] : null;
+
+        $headers = [
+            McpHeader::PROTOCOL_VERSION => $protocolVersion->value,
+            McpHeader::METHOD => $method,
+        ];
+
+        if (null !== $name = McpHeader::nameFor($method, $params)) {
+            // Tool and prompt names are only SHOULD-constrained to header-safe
+            // characters and a resource URI is not constrained at all, so
+            // anything unsafe is wrapped rather than dropped or mangled.
+            $headers[McpHeader::NAME] = McpHeader::encode($name) ?? $name;
+        }
+
+        if ('tools/call' === $method && \is_string($params['name'] ?? null)) {
+            $arguments = \is_array($params['arguments'] ?? null) ? $params['arguments'] : [];
+
+            foreach ($this->tools->headersFor($params['name'], $arguments) as $suffix => $value) {
+                $headers[McpHeader::PARAM_PREFIX.$suffix] = $value;
+            }
+        }
+
+        return $headers;
+    }
+}