[Server] Throw the SDK's own RuntimeException from FileSessionStore (#404)

The constructor guarded its unwritable-directory case with the global
`\RuntimeException`, the only throw in `src/` still outside
`Mcp\Exception\ExceptionInterface`. A consumer wrapping server setup in a
`catch (ExceptionInterface)` missed it and got an unhandled SPL exception on
a misconfigured session directory instead.

`Mcp\Exception\RuntimeException` extends `\RuntimeException`, so this is
additive: existing `catch (\RuntimeException)` blocks keep working.

The constructor had no test coverage at all, so this adds one for the throw
plus the directory-creation and write/read paths around it.

src/Server/Session/FileSessionStore.php

@@ -11,6 +11,7 @@
 
 namespace Mcp\Server\Session;
 
+use Mcp\Exception\RuntimeException;
 use Mcp\Server\NativeClock;
 use Psr\Clock\ClockInterface;
 use Symfony\Component\Uid\Uuid;
@@ -31,7 +32,7 @@ class FileSessionStore implements SessionStoreInterface
         }
 
         if (!is_dir($this->directory) || !is_writable($this->directory)) {
-            throw new \RuntimeException(\sprintf('Session directory "%s" is not writable.', $this->directory));
+            throw new RuntimeException(\sprintf('Session directory "%s" is not writable.', $this->directory));
         }
     }
 

tests/Unit/Server/Session/FileSessionStoreTest.php

@@ -0,0 +1,90 @@
+<?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\ExceptionInterface;
+use Mcp\Exception\RuntimeException;
+use Mcp\Server\Session\FileSessionStore;
+use PHPUnit\Framework\Attributes\TestDox;
+use PHPUnit\Framework\TestCase;
+use Symfony\Component\Uid\UuidV4;
+
+class FileSessionStoreTest extends TestCase
+{
+    private string $directory;
+
+    protected function setUp(): void
+    {
+        $this->directory = sys_get_temp_dir().'/mcp-file-session-store-'.bin2hex(random_bytes(6));
+    }
+
+    protected function tearDown(): void
+    {
+        if (!is_dir($this->directory)) {
+            return;
+        }
+
+        @chmod($this->directory, 0775);
+
+        foreach (glob($this->directory.'/*') ?: [] as $file) {
+            @unlink($file);
+        }
+
+        @rmdir($this->directory);
+    }
+
+    #[TestDox('creates the session directory when it does not exist yet')]
+    public function testCreatesMissingDirectory(): void
+    {
+        new FileSessionStore($this->directory);
+
+        $this->assertDirectoryExists($this->directory);
+    }
+
+    #[TestDox('round-trips a session payload through the filesystem')]
+    public function testWriteThenRead(): void
+    {
+        $store = new FileSessionStore($this->directory);
+        $id = new UuidV4();
+
+        $store->write($id, 'payload');
+
+        $this->assertTrue($store->exists($id));
+        $this->assertSame('payload', $store->read($id));
+    }
+
+    #[TestDox('rejects an unwritable directory with the SDK\'s own exception')]
+    public function testUnwritableDirectoryThrowsPackageException(): void
+    {
+        mkdir($this->directory, 0775, true);
+        chmod($this->directory, 0555);
+        clearstatcache(true, $this->directory);
+
+        if (is_writable($this->directory)) {
+            $this->markTestSkipped('Permission bits do not restrict writes here (running as root, or a filesystem that ignores them).');
+        }
+
+        // The store's only throw must stay inside the package hierarchy, so a
+        // consumer catching ExceptionInterface sees it rather than a bare SPL
+        // RuntimeException escaping the SDK.
+        $this->expectException(RuntimeException::class);
+        $this->expectExceptionMessage(\sprintf('Session directory "%s" is not writable.', $this->directory));
+
+        try {
+            new FileSessionStore($this->directory);
+        } catch (RuntimeException $e) {
+            $this->assertInstanceOf(ExceptionInterface::class, $e);
+
+            throw $e;
+        }
+    }
+}