fix: mark client initialized after initialize handshake

Many real-world MCP clients (Claude Code, Gemini/Antigravity)
complete the initialize handshake and immediately call tools/list or
tools/call without sending the notifications/initialized notification.

Currently, initialized is only set to true in
handleNotificationInitialized(), so these clients get permanently
locked out with "Client session not initialized" errors.

This change marks the session as initialized right after a successful
initialize response. If a client still sends notifications/initialized,
handleNotificationInitialized() sets the flag again — a harmless no-op.

Discovered while running a production MCP server (Cohete blog) that
serves Claude Code and other MCP clients over SSE transport.

Author: pascualmg

src/Dispatcher.php

@@ -128,6 +128,12 @@ public function handleInitialize(InitializeRequest $request, SessionInterface $s
         $session->set('client_info', $request->clientInfo->toArray());
         $session->set('protocol_version', $protocolVersion);
 
+        // Mark client as initialized immediately after successful handshake.
+        // Many real-world MCP clients (Claude Code, Gemini) skip
+        // the notifications/initialized step, causing "Client session not
+        // initialized" errors on all subsequent requests.
+        $session->set('initialized', true);
+
         $serverInfo = $this->configuration->serverInfo;
         $capabilities = $this->configuration->capabilities;
         $instructions = $this->configuration->instructions;