refactor: Add interface to final readonly chain for testability (#144)

Author: DZunke

src/Chain.php

@@ -14,7 +14,7 @@
 use PhpLlm\LlmChain\Message\MessageBag;
 use PhpLlm\LlmChain\Response\ResponseInterface;
 
-final readonly class Chain
+final readonly class Chain implements ChainInterface
 {
     /**
      * @var InputProcessor[]

src/ChainInterface.php

@@ -0,0 +1,16 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PhpLlm\LlmChain;
+
+use PhpLlm\LlmChain\Message\MessageBag;
+use PhpLlm\LlmChain\Response\ResponseInterface;
+
+interface ChainInterface
+{
+    /**
+     * @param array<string, mixed> $options
+     */
+    public function call(MessageBag $messages, array $options = []): ResponseInterface;
+}