Add interface abstractions for SchemaGenerator and Discoverer with DI improvements (#215)

* schema generator injection

* add interface for schema generator

* improve discoverer dependency

* remove some comments

* make some classes final

* simplify condition

* code review

* Extend changelog for covering interfaces

* Adopt SchemaGeneratorInterface after rebase and #153

---------

Co-authored-by: Christopher Hertel <mail@christopher-hertel.de>

src/Capability/Discovery/DiscovererInterface.php

@@ -0,0 +1,31 @@
+<?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\Capability\Discovery;
+
+/**
+ * Discovers MCP elements (tools, resources, prompts, resource templates) in directories.
+ *
+ * @internal
+ *
+ * @author Antoine Bluchet <soyuka@gmail.com>
+ */
+interface DiscovererInterface
+{
+    /**
+     * Discover MCP elements in the specified directories and return the discovery state.
+     *
+     * @param string        $basePath    the base path for resolving directories
+     * @param array<string> $directories list of directories (relative to base path) to scan
+     * @param array<string> $excludeDirs list of directories (relative to base path) to exclude from the scan
+     */
+    public function discover(string $basePath, array $directories, array $excludeDirs = []): DiscoveryState;
+}