[Server] Shift away session decode from constructor (#238)

* Shift away session decode from constructor

* add a small test

---------

Co-authored-by: Tobias Nyholm <tobias.nyholm@gmail.com>

tests/Unit/Server/Session/SessionTest.php

@@ -0,0 +1,36 @@
+<?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\Tests\Unit\Server\Session;
+
+use Mcp\Server\Session\InMemorySessionStore;
+use Mcp\Server\Session\Session;
+use PHPUnit\Framework\TestCase;
+
+class SessionTest extends TestCase
+{
+    public function testAll()
+    {
+        $store = $this->getMockBuilder(InMemorySessionStore::class)
+            ->disableOriginalConstructor()
+            ->onlyMethods(['read'])
+            ->getMock();
+        $store->expects($this->once())->method('read')->willReturn(json_encode(['foo' => 'bar']));
+
+        $session = new Session($store);
+        $result = $session->all();
+        $this->assertEquals(['foo' => 'bar'], $result);
+
+        // Call again to make sure we dont read from Store
+        $result = $session->all();
+        $this->assertEquals(['foo' => 'bar'], $result);
+    }
+}