Add configurable session garbage collection (gcProbability/gcDivisor) (#258)

Make session GC probability configurable via gcProbability and gcDivisor
parameters, mirroring PHP's session.gc_probability/session.gc_divisor.
Exposed through both SessionManager constructor and Builder::setSession().

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

tests/Unit/Server/Session/SessionManagerTest.php

@@ -0,0 +1,80 @@
+<?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\Exception\InvalidArgumentException;
+use Mcp\Server\Session\InMemorySessionStore;
+use Mcp\Server\Session\SessionManager;
+use PHPUnit\Framework\TestCase;
+
+class SessionManagerTest extends TestCase
+{
+    public function testGcDisabledWhenProbabilityIsZero(): void
+    {
+        $store = $this->createMock(InMemorySessionStore::class);
+        $store->expects($this->never())->method('gc');
+
+        $manager = new SessionManager($store, gcProbability: 0);
+
+        // Call gc many times — it should never trigger
+        for ($i = 0; $i < 100; ++$i) {
+            $manager->gc();
+        }
+    }
+
+    public function testGcAlwaysRunsWhenProbabilityEqualsDivisor(): void
+    {
+        $store = $this->createMock(InMemorySessionStore::class);
+        $store->expects($this->exactly(10))->method('gc')->willReturn([]);
+
+        $manager = new SessionManager($store, gcProbability: 1, gcDivisor: 1);
+
+        for ($i = 0; $i < 10; ++$i) {
+            $manager->gc();
+        }
+    }
+
+    public function testGcAlwaysRunsWhenProbabilityExceedsDivisor(): void
+    {
+        $store = $this->createMock(InMemorySessionStore::class);
+        $store->expects($this->exactly(5))->method('gc')->willReturn([]);
+
+        $manager = new SessionManager($store, gcProbability: 100, gcDivisor: 1);
+
+        for ($i = 0; $i < 5; ++$i) {
+            $manager->gc();
+        }
+    }
+
+    public function testGcProbabilityMustBeNonNegative(): void
+    {
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('gcProbability must be greater than or equal to 0.');
+
+        new SessionManager(new InMemorySessionStore(), gcProbability: -1);
+    }
+
+    public function testGcDivisorMustBePositive(): void
+    {
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('gcDivisor must be greater than or equal to 1.');
+
+        new SessionManager(new InMemorySessionStore(), gcDivisor: 0);
+    }
+
+    public function testDefaultGcConfiguration(): void
+    {
+        // Default should be 1/100 — just verify construction works
+        $manager = new SessionManager(new InMemorySessionStore());
+        $this->assertInstanceOf(SessionManager::class, $manager);
+    }
+}